github actions: do not use 20.04

[tjhei]

part of #6231

[tjhei]

I opted to do one at 24.04 (shipping 9.5) and one at 22.04. We might also think about updating to 9.6: maybe just the no-unity build or something?

[tjhei]

I am running into biber warnings breaking the documentation build on 22.04:
Use of uninitialized value $opt in hash element at /usr/share/perl5/Biber/Config.pm line 967.

[gassmoeller]

/rebuild

[gassmoeller]

let us know if you need testing and/or a review

I think running one tester on 22.04 and one on 24.04 is a good idea. Switching to deal.II 9.6 was on my list of things to do any time before this years hackathon, so we can remove some compatibility code as starter projects. So yes, moving the testers to 9.6 also makes sense.

[tjhei]

I think this works and is good to go. We can change deal.II versions separately.

Are you saying we should require 9.6 soon?