CI should mostly be on Debian 12 'bookworm', with maybe one job using Debian 11 'bullseye'
Debian 12 'bookworm' was released as stable in mid 2023, so we shouldn't be trying too hard to target anything older than that. Newer compilers will probably give us better diagnostics.
Let's get the CI green again before we do anything in this direction, but after CI is working, it would be good to get the Debian-based parts of our CI running on bookworm instead of the current bullseye.
Optionally, we could have one job that runs on bullseye, to verify that we're at least somewhat backwards-compatible (the same way we had a Debian 10 'buster' job before it was removed in 5d192278).