我目前正在运行 14.04 LTS Ubuntu 服务器，我想将其升级到 16.04。我已经安装了该update-manager-core工具并将其设置为搜索 LTS 版本，但它每次都返回“未找到新版本”。
daniel@Owncloud:~$ sudo do-release-upgrade -d -s
Checking for a new Ubuntu release
No new release found
daniel@Owncloud:~$ cat /etc/update-manager/release-upgrades
# Default behavior for the release upgrader.
[DEFAULT]
# Default prompting behavior, valid options:
#
# never - Never check for a new release.
# normal - Check to see if a new release is available. If more than one new
# release is found, the release upgrader will attempt to upgrade to
# the release that immediately succeeds the currently-running
# release.
# lts - Check to see if a new LTS release is available. The upgrader
# will attempt to upgrade to the first LTS release available after
# the currently-running one. Note that this option should not be
# used if the currently-running release is not itself an LTS
# release, since in that case the upgrader won't be able to
# determine if a newer release is available.
Prompt=lts