我的电脑上有 Ubuntu Mate 18.04：
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.3 LTS
Release: 18.04
Codename: bionic
当我在终端中运行时：
sudo apt-get update
我得到：
Ign:1 http://old-releases.ubuntu.com/ubuntu bionic InRelease
Ign:2 http://old-releases.ubuntu.com/ubuntu bionic-updates InRelease
Ign:3 http://old-releases.ubuntu.com/ubuntu bionic-security InRelease
Err:4 http://old-releases.ubuntu.com/ubuntu bionic Release
404 Not Found [IP: 91.189.88.153 80]
Err:5 http://old-releases.ubuntu.com/ubuntu bionic-updates Release
404 Not Found [IP: 91.189.88.153 80]
Err:6 http://old-releases.ubuntu.com/ubuntu bionic-security Release
404 Not Found [IP: 91.189.88.153 80]
Reading package lists... Done
E: The repository 'http://old-releases.ubuntu.com/ubuntu bionic Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://old-releases.ubuntu.com/ubuntu bionic-updates Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://old-releases.ubuntu.com/ubuntu bionic-security Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
这是我的sources.list：
deb http://old-releases.ubuntu.com/ubuntu/ bionic main restricted
deb http://old-releases.ubuntu.com/ubuntu/ bionic-updates main restricted
deb http://old-releases.ubuntu.com/ubuntu/ bionic universe
deb http://old-releases.ubuntu.com/ubuntu/ bionic-updates universe
deb http://old-releases.ubuntu.com/ubuntu/ bionic multiverse
deb http://old-releases.ubuntu.com/ubuntu/ bionic-updates multiverse
deb http://old-releases.ubuntu.com/ubuntu bionic-security main restricted
deb http://old-releases.ubuntu.com/ubuntu bionic-security universe
deb http://old-releases.ubuntu.com/ubuntu bionic-security multiverse
我该如何解决这个问题？我无法从存储库安装程序，也无法更新到 Ubuntu Mate 20.04 的新版本。
任何帮助都将受到赞赏。
谢谢