Support Debian 13

[hax0rbana-adam]

This is the merge request from #597 but with the conflicts resolved, accepting the suggested changes, added a missing dependency for using deb822_repository and removing the default_release attribute that causes "invalid APT::Default-Release" errors.

[otakup0pe]

Some feedback; I believe there are still people using this role on focal.... And I'm not seeing postgresql_apt_repository_url referenced anywhere?

[hax0rbana-adam]

Thank you for the heads up about Ubuntu 20.04 (focal). It was still in the readme and the molecule tests in this merge request, but that was because it was inadvertently trying to bring back the 20.04 support. That was the result of an incorrect merge conflict resolution on my part. Focal support was dropped on the default branch.

Because this merge request did not intend to affect 20.04 support, I've updated this README and molecule tests to match the default branch.

The question of whether focal support should be maintained for Focal is valid (as Focal is still supported for those who pay for a support contract), but it is beyond the scope of this MR.

postgresql_apt_repository_url is used in tasks/install_apt.yml.

[github-actions]

This pr has been marked 'stale' due to lack of recent activity. If there is no further activity, the issue will be closed in another 30 days. Thank you for your contribution!

[hax0rbana-adam]

comment to tell the bot that this is still a fix that is waiting to be merged

[otakup0pe]

need conflicts resolved and tests passing before we merge.

[hax0rbana-adam]

Resolved conflicts and all tests should pass now.

This change is now more limited in scope because someone already duplicated the work of removing apt_key in the default branch and the default branch works in Debian 13 out of the box. This MR has been cut down to just make it official that this repo now supports Debian 13. This means updating the CI jobs to include it, the documentation, and I left in changes like having the repo url be a variable instead of hard-coding it with no ability to override it.