CI: minimal dependency versions: specify them and try them in CI

[pajod]

We should know our minimum versions, and possibly how they are different depending on python version, e.g. see #2675

This is not the change in minimal dependencies, this is just the necessary duplication (https://github.com/pypa/pip/issues/8085) and hooking it up to GitHub CI. Has everything added as comments though, so after this one is merged, any future PR actually bumping dependencies could have that extra ✅/❌ show up.

The useless header/comment line swap is intentional, works around a bug in GitHub diff syntax highlighting.

[pajod]

Attached further patches:

• CI should verify that whatever changes and workarounds are applied by debhelper do not break building
  • the best way to do that would be to add Debian metadata and call dpkg-buildpackage -us -uc
    • note: Debian metadata used for this purpose shall not end up in source tarballs
  • this one is not quite complete, as I have not been able to instruct setuptools 59.6.0 (from 2021) to read our toml-format metadata, so the CI job falls back to copying a minimal cfg-format to at least try the remaining steps so I use the new ubuntu-24.04 worker instead
• CI should verify that python -m build works
  • whether that is done for publishing on a regular basis or not, using the pypi-provided github action seems reasonable
    • DANGER this is prepared in my patch, but partly commented out as it is non-obvious how to structure the CI configuration without running into one of the "oops, everyone with permission to open PR has write access" traps GitHub has conveniently laid out