我正在使用 Ubuntu 20.04.3 LTS。我已经遇到错误 2 天了，即由于以下错误，我无法在系统上更新或升级或执行软件包安装：
Hit:1 http://packages.microsoft.com/repos/code stable InRelease
Hit:2 http://archive.canonical.com/ubuntu focal InRelease
Hit:3 http://ppa.launchpad.net/deadsnakes/ppa/ubuntu focal InRelease
Hit:4 http://dl.google.com/linux/chrome/deb stable InRelease
Ign:5 http://old-releases.ubuntu.com/ubuntu focal InRelease
Ign:6 http://old-releases.ubuntu.com/ubuntu focal-updates InRelease
Hit:7 http://ppa.launchpad.net/kicad/kicad-5.1-releases/ubuntu focal InRelease
Hit:8 https://deb.opera.com/opera-stable stable InRelease
Hit:9 https://packages.microsoft.com/repos/vscode stable InRelease
Ign:10 http://old-releases.ubuntu.com/ubuntu focal-backports InRelease
Ign:11 http://ppa.launchpad.net/team-gcc-arm-embedded/ppa/ubuntu focal InRelease
Ign:12 http://old-releases.ubuntu.com/ubuntu focal-security InRelease
Err:14 http://old-releases.ubuntu.com/ubuntu focal Release
404 Not Found [IP: 91.189.88.247 80]
Err:15 http://ppa.launchpad.net/team-gcc-arm-embedded/ppa/ubuntu focal Release
404 Not Found [IP: 91.189.95.85 80]
Err:16 http://old-releases.ubuntu.com/ubuntu focal-updates Release
404 Not …