[RFC] Move GitLab testing to Ubuntu 22.04

[Jonas Hahnfeld]

Hi all,

I mentioned this during the transition to Guile 3.0, and now I had the
time to prepare the required changes to move the base of our GitLab
testing to Ubuntu 22.04. The merge request to do so is here:
https://gitlab.com/lilypond/lilypond/-/merge_requests/2318

There is one caveat here that I would like to mention: During initial
testing in my fork, one test job unexpectedly hang during a lilypond
execution, using 100% CPU and eventually being killed after 60 minutes.
Subsequent runs were fine, so this needs to be monitored and
investigated if it happens again...

As noted in the MR summary, switching to Ubuntu 22.04 will give us
testing with the more recent Guile 3.0.7, and I would like to bump the
requirement to that version, after the switch had some time to settle.
Likewise I think we should bump the requirement to Texinfo 6.8 (which
will allow us to drop quite some compatibility code in lilypond.init)
and Python 3.10 (because Python scripts are notoriously easy to break
if only tested with more recent interpreter versions). The only
downside of this is that LilyPond master will not compile anymore on
older distributions, including Ubuntu 20.04. I personally think this is
fine, for example Ubuntu 24.04 was just released, but let me know if
you disagree.

Cheers
Jonas

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