As you note, Buster is no longer supported. Obviously the best way to resolve that is move to a supported release.

But in the meantime, you could just disable the Debian repos altogether by commenting out the relevant lines (add a '#' prefix to each line) so it wouldn't try to connect at all.

However, the Buster repos still exist, they've just been archived - moved from "deb.debian.org" to "archive.debian.org". So instead of disabling them completely, I recommend updating them to the archive subdomain. There still won't be any updates, but it will remove the error. Plus as a bonus it will allow you to install any additional software that you may want in the future. I imagine that's probably unlikely but might come in handy?

As you've possibly guessed, it's as simple as updating the relevant lines in the relevant apt sources files. FYI the 'security.debian.org' domain should only be in '/etc/apt/sources.list.d/security.sources.list' and the 'deb.debian.org' in '/etc/apt/sources.list.d/sources.list'. You can do that manually with a text editor, or semi-automatically with these commands:

sed -i.bak "s|security.debian.org|archive.debian.org|g" /etc/apt/sources.list.d/security.sources.list
sed -i.bak "s|deb.debian.org|archive.debian.org|g" /etc/apt/sources.list.d/sources.list

Those 2 commands should "just work" but worse case scenario, you can find backups of the 2 edited files in '/etc/apt/sources.list.d/' - same filename as the originals but with a '.bak' suffix. Renaming them and removing the '.bak' file extension will get you back to where you are now. Make sure that you only run those commands once though. Rerunning them will overwrite the backup '.bak' files.

After the updates, re-run 'apt-get update' and the errors should be gone. It may warn that it's skipping those backup files, but it shouldn't error. Once you've confirmed that it all works as expected, you can then delete the backup ('.bak') files.