Fix release not found in repository when running sudo apt update

linux-fix-apt-update.md

If running update on a no longer supported Ubuntu version (ie. 19.10):

justin@pop-os:~$ sudo apt update
Ign:1 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:2 http://apt.pop-os.org/proprietary disco InRelease                                                           
Get:3 http://repository.spotify.com stable InRelease [3,316 B]                                                    
Hit:4 http://dl.google.com/linux/chrome/deb stable Release                                                        
Hit:5 http://packages.microsoft.com/repos/vscode stable InRelease                                                 
Ign:6 http://archive.ubuntu.com/ubuntu disco InRelease                                                            
Ign:7 http://archive.ubuntu.com/ubuntu disco-updates InRelease                                                    
Hit:8 http://ppa.launchpad.net/system76/pop/ubuntu disco InRelease                                         
Ign:9 http://archive.ubuntu.com/ubuntu disco-backports InRelease                     
Err:10 http://archive.ubuntu.com/ubuntu disco Release
  404  Not Found [IP: 2001:67c:1562::15 80]
Ign:11 http://security.ubuntu.com/ubuntu disco-security InRelease
Err:12 http://archive.ubuntu.com/ubuntu disco-updates Release
  404  Not Found [IP: 2001:67c:1562::15 80]
Err:14 http://security.ubuntu.com/ubuntu disco-security Release
  404  Not Found [IP: 2001:67c:1360:8001::24 80]
Err:15 http://archive.ubuntu.com/ubuntu disco-backports Release
  404  Not Found [IP: 2001:67c:1562::15 80]
Reading package lists... Done
E: The repository 'http://archive.ubuntu.com/ubuntu disco 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://archive.ubuntu.com/ubuntu disco-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://security.ubuntu.com/ubuntu disco-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.
E: The repository 'http://archive.ubuntu.com/ubuntu disco-backports 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.

Edit /etc/apt/sources.list from archive.ubuntu.com:

# deb cdrom:[Pop_OS 19.04 _Disco Dingo_ - Release amd64 (20190806)]/ disco main restricted
deb http://archive.ubuntu.com/ubuntu/ disco restricted universe multiverse main
deb http://apt.pop-os.org/proprietary disco main
# deb-src http://apt.pop-os.org/proprietary disco main

deb http://archive.ubuntu.com/ubuntu/ disco-updates multiverse universe restricted main
deb http://security.ubuntu.com/ubuntu/ disco-security multiverse universe restricted main
deb http://archive.ubuntu.com/ubuntu/ disco-backports multiverse universe restricted main

To old-releases.ubuntu.com:

# deb cdrom:[Pop_OS 19.04 _Disco Dingo_ - Release amd64 (20190806)]/ disco main restricted
deb http://old-releases.ubuntu.com/ubuntu/ disco restricted universe multiverse main
deb http://apt.pop-os.org/proprietary disco main
# deb-src http://apt.pop-os.org/proprietary disco main

deb http://old-releases.ubuntu.com/ubuntu/ disco-updates multiverse universe restricted main
deb http://old-releases.ubuntu.com/ubuntu/ disco-security multiverse universe restricted main
deb http://old-releases.ubuntu.com/ubuntu/ disco-backports multiverse universe restricted main