CI build Linux directly on Ubuntu 22

[softins]

Short description of changes

CI build the Linux packages directly on an Ubuntu 22 runner, instead of using an Ubuntu 20 container.

• The container frequently fails to fetch all its dependencies, causing random build failures.
• Ubuntu-20 builds with Qt 5.12, but Ubuntu-22 builds with Qt 5.15
• This drops support for installing the deb on Debian 11 or Ubuntu 20; minimum versions are Debian 12 or Ubuntu 22.
• Users can still build from source to support Debian 11 or Ubuntu 20 if desired.

CHANGELOG: Build: use Ubuntu 22 for building Linux deb packages.

Context: Fixes an issue?

Should result in more reliable CI builds.

Does this change need documentation? What needs to be documented and how?

We should mention somewhere that the minimum supported Linux versions for the release packages are Debian 12 or Ubuntu 22.

Status of this Pull Request

Ready for review

What is missing until this pull request can be merged?

Nothing

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

AUTOBUILD: Please build all targets

[ann0see]

Will need to check on my Debian device then. I did some tests with Qt6 but it seemed that it broke Debian stable builds for me. Eventually we'll need to update to Qt6 based Linux builds anyway.

[dingodoppelt]

I just noticed we were building on different ubuntu versions when I tried to fix my autobuilds. Thanks for your fix!
It looks like it's working as intended: all builds on ubuntu runners have completed successfully. That's enough for me :)

[ann0see]

Still, I'll want to test it as we had weird incompatibilities a while ago. Also we may need to update the deb packet dependencies.

[softins]

I verified the built deb package installed correctly on Debian 12, Debian 13, Ubuntu 22 and Ubuntu 24.