我正在运行 Ubuntu MATE 18.04 LTS。一两周前，软件更新工具开始显示以下消息：
并非所有更新都可安装。运行部分升级以安装尽可能多的更新。
这可能是由于以下原因造成的：[...]
当我单击“继续”时，它会尝试启动分发升级，但我目前已取消该升级。
我只启用了新 LTS 版本（一年多后才会到期）的发行版升级，因此我想知道这里应该进行哪种发行版升级。更新程序没有提到新发行版是什么。
这是怎么回事？我怎样才能恢复正常的系统更新，而不会冒损坏任何内容的风险（或升级到我不想要的发行版）？
编辑：
输出自apt update：
Hit:1 http://archive.canonical.com/ubuntu bionic InRelease
Hit:2 http://ubuntu.uni-klu.ac.at/ubuntu bionic InRelease
Get:3 http://ubuntu.uni-klu.ac.at/ubuntu bionic-updates InRelease [88.7 kB]
Hit:4 https://apt.syncthing.net syncthing InRelease
Get:5 http://ubuntu.uni-klu.ac.at/ubuntu bionic-security InRelease [83.2 kB]
Get:6 http://ubuntu.uni-klu.ac.at/ubuntu bionic-updates/main i386 Packages [404 kB]
Get:7 http://ubuntu.uni-klu.ac.at/ubuntu bionic-updates/main amd64 Packages [460 kB]
Get:8 http://ubuntu.uni-klu.ac.at/ubuntu bionic-updates/universe amd64 Packages [595 kB]
Get:9 http://ubuntu.uni-klu.ac.at/ubuntu bionic-updates/universe i386 Packages [589 kB]
Fetched 2,220 kB in 10s (222 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
396 packages can be upgraded. Run 'apt list --upgradable' to see them.
输出自apt upgrade：
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages were automatically installed and are no longer required:
geoip-database-extra gir1.2-gmenu-3.0 libaccounts-glib0 libaccounts-qt5-1 libgnome-menu-3-0 libjs-coffeescript libjs-openlayers
libsignon-extension1 libsignon-glib1 libsignon-plugins-common1 libsignon-qt5-1 libudev1:i386 libwireshark10 libwiretap7
libwnck-common libwnck22 libwscodecs1 libwsutil8 python-aptdaemon python-aptdaemon.gtk3widgets python-attr python-automat
python-click python-colorama python-constantly python-debian python-defer python-dirspec python-hyperlink python-incremental
python-piston-mini-client python-pyasn1 python-twisted-bin python-xapian python3-blinker python3-jwt python3-oauthlib
python3-piston-mini-client ruby-afm ruby-ascii85 ruby-blankslate ruby-classifier-reborn ruby-coderay ruby-coffee-script
ruby-coffee-script-source ruby-colorator ruby-execjs ruby-fast-stemmer ruby-ffi ruby-hashery ruby-jekyll-coffeescript
ruby-jekyll-feed ruby-jekyll-gist ruby-jekyll-paginate ruby-jekyll-sass-converter ruby-jekyll-watch ruby-json ruby-kramdown
ruby-launchy-shim ruby-liquid ruby-listen ruby-mercenary ruby-mime-types ruby-mime-types-data ruby-multi-json ruby-oj ruby-parslet
ruby-pdf-core ruby-pdf-reader ruby-pg ruby-posix-spawn ruby-prawn ruby-prawn-table ruby-pygments.rb ruby-rb-inotify ruby-rc4
ruby-rdiscount ruby-redcarpet ruby-rouge ruby-safe-yaml ruby-sass ruby-sequel ruby-sequel-pg ruby-stringex ruby-toml ruby-ttfunk
ruby-yajl signon-plugin-oauth2 signon-ui-service signon-ui-x11 signond software-center-aptdaemon-plugins unity-asset-pool
Use 'sudo apt autoremove' to remove them.
The following NEW packages will be installed:
bubblewrap libgles1 libmaxminddb0 libqgsttools-p1 libqt5multimedia5-plugins libqt5multimediawidgets5 libwireshark11 libwiretap8
libwscodecs2 libwsutil9 linux-headers-4.15.0-42 linux-headers-4.15.0-42-generic linux-image-4.15.0-42-generic
linux-modules-4.15.0-42-generic linux-modules-extra-4.15.0-42-generic linux-tools-4.15.0-42 linux-tools-4.15.0-42-generic
python3-netifaces
The following packages have been kept back:
libssl-dev libssl1.1 nodejs nodejs-dev
The following packages will be upgraded:
apparmor apport apport-gtk apt apt-transport-https apt-utils apturl apturl-common autopoint bind9-host binutils binutils-common
binutils-x86-64-linux-gnu bsdutils chromium-codecs-ffmpeg-extra clamav clamav-base clamav-freshclam clamdscan cpp cpp-7 cups
cups-bsd cups-client cups-common cups-core-drivers cups-daemon cups-ipp-utils cups-ppdc cups-server-common curl desktop-file-utils
distro-info-data dnsutils dpkg dpkg-dev enigmail fdisk firefox firefox-locale-en flashplugin-installer fonts-liberation
fonts-liberation2 fonts-noto-color-emoji fonts-opensymbol friendly-recovery fwupd g++ g++-7 gcc gcc-7 gcc-7-base gcc-8-base
gcc-8-base:i386 gettext gettext-base ghostscript ghostscript-x gir1.2-javascriptcoregtk-4.0 gir1.2-packagekitglib-1.0
gir1.2-snapd-1 gir1.2-totem-1.0 gir1.2-udisks-2.0 gir1.2-webkit2-4.0 git git-gui git-man git-svn gitk gnome-desktop3-data
gnome-settings-daemon-schemas grub-common grub-pc grub-pc-bin grub2-common hwdata imagemagick imagemagick-6-common
imagemagick-6.q16 initramfs-tools initramfs-tools-bin initramfs-tools-core kmod libapparmor-perl libapparmor1 libappstream4
libapt-inst2.0 libapt-pkg5.0 libasan4 libatomic1 libbind9-160 libbinutils libblkid1 libblkid1:i386 libbrotli1 libcc1-0 libcilkrts5
libclamav7 libconfuse-common libconfuse2 libcups2 libcups2:i386 libcupscgi1 libcupsimage2 libcupsmime1 libcupsppdc1
libcurl3-gnutls libcurl4 libdns-export1100 libdns1100 libdpkg-perl libegl1 libfdisk1 libfreerdp-client2-2 libfreerdp2-2 libfwupd2
libgcc-7-dev libgcc1 libgcc1:i386 libgfortran4 libgl1 libgl1:i386 libgles2 libglib2.0-0 libglib2.0-0:i386 libglib2.0-bin
libglib2.0-data libglib2.0-dev libglib2.0-dev-bin libglvnd-core-dev libglvnd-dev libglvnd0 libglvnd0:i386 libglx0 libglx0:i386
libgnome-desktop-3-17 libgomp1 libgs9 libgs9-common libimage-magick-perl libimage-magick-q16-perl libirs160 libisc-export169
libisc169 libisccc160 libisccfg160 libitm1 libjavascriptcoregtk-4.0-18 libkmod2 libkpathsea6 libldap-2.4-2 libldap-common
liblouis-data liblouis14 liblsan0 liblwres160 libmad0 libmagick++-6.q16-7 libmagickcore-6.q16-3 libmagickcore-6.q16-3-extra
libmagickwand-6.q16-3 libmetacity1 libmount1 libmount1:i386 libmpx2 libmspack0 libmysqlclient20 libmysqlclient20:i386
libnautilus-extension1a libnm0 libnss-systemd libobjc4 libopengl0 libpackagekit-glib2-18 libpam-systemd libparted-fs-resize0
libparted2 libperl5.26 libplymouth4 libpoppler-glib8 libpoppler73 libpq-dev libpq5 libptexenc1 libpython2.7 libpython2.7-dev
libpython2.7-minimal libpython2.7-stdlib libpython3-dev libpython3-stdlib libpython3.6 libpython3.6-dev libpython3.6-minimal
libpython3.6-stdlib libquadmath0 libraw16 libreoffice-avmedia-backend-gstreamer libreoffice-base-core libreoffice-calc
libreoffice-common libreoffice-core libreoffice-draw libreoffice-gnome libreoffice-gtk3 libreoffice-help-en-us libreoffice-impress
libreoffice-math libreoffice-ogltrans libreoffice-pdfimport libreoffice-style-galaxy libreoffice-style-human
libreoffice-style-tango libreoffice-writer libruby2.5 libsmartcols1 libsmbclient libsnapd-glib1 libsnmp-base libsnmp30 libssh-4
libssh-gcrypt-4 libssl-doc libssl1.0.0 libssl1.0.0:i386 libstdc++-7-dev libstdc++6 libstdc++6:i386 libsynctex1 libsystemd0
libsystemd0:i386 libtexlua52 libtexluajit2 libtotem0 libtsan0 libubsan0 libudev-dev libudev1 libudev1:i386 libudisks2-0 libuuid1
libuuid1:i386 libvlc-bin libvlc5 libvlccore9 libwavpack1 libwbclient0 libwebkit2gtk-4.0-37 libwinpr2-2 libwireshark-data libwoff1
libxkbcommon-dev libxkbcommon-x11-0 libxkbcommon0 linux-firmware linux-generic linux-headers-generic linux-image-generic
linux-libc-dev linux-tools-common linux-tools-virtual man-db metacity-common module-init-tools mount netplan.io network-manager
networkd-dispatcher nodejs-doc nplan nvidia-prime openjdk-11-jdk openjdk-11-jdk-headless openjdk-11-jre openjdk-11-jre-headless
openjdk-8-jdk openjdk-8-jdk-headless openjdk-8-jre openjdk-8-jre-headless openssh-client openssh-server openssh-sftp-server
openssl packagekit packagekit-tools parted perl perl-base perl-modules-5.26 plymouth plymouth-label plymouth-theme-ubuntu-text
poppler-utils ppp python-apport python-apt python-apt-common python-lxml python-problem-report python-requests python-samba
python2.7 python2.7-dev python2.7-minimal python3 python3-apport python3-apt python3-dev python3-distupgrade python3-distutils
python3-gdbm python3-lib2to3 python3-louis python3-minimal python3-problem-report python3-requests python3-software-properties
python3-tk python3-update-manager python3.6 python3.6-dev python3.6-minimal rfkill ruby2.5 ruby2.5-dev ruby2.5-doc samba-common
samba-common-bin samba-libs secureboot-db shotwell shotwell-common smbclient software-properties-common software-properties-gtk
ssh syncthing systemd systemd-sysv texlive-binaries thunderbird thunderbird-locale-en thunderbird-locale-en-us totem totem-common
totem-plugins tzdata ubuntu-drivers-common ubuntu-keyring ubuntu-mono ubuntu-release-upgrader-core ubuntu-release-upgrader-gtk
udev udisks2 ufraw-batch unattended-upgrades uno-libs3 update-manager update-manager-core update-notifier update-notifier-common
ure usbmuxd util-linux uuid-dev uuid-runtime valgrind virtualbox virtualbox-dkms virtualbox-qt vlc vlc-bin vlc-data vlc-l10n
vlc-plugin-base vlc-plugin-notify vlc-plugin-qt vlc-plugin-samba vlc-plugin-skins2 vlc-plugin-video-output
vlc-plugin-video-splitter vlc-plugin-visualization wavpack wireshark wireshark-common wireshark-qt xdg-utils xserver-common
xserver-xorg-core xserver-xorg-legacy xul-ext-calendar-timezones xul-ext-gdata-provider xul-ext-lightning
392 upgraded, 18 newly installed, 0 to remove and 4 not upgraded.
Need to get 789 MB of archives.
After this operation, 539 MB of additional disk space will be used.
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