Remove Ubuntu 20.04 Focal from CI/CD (`x86-64_musl` jobs left)

[kelson42]

Reasons are:

• Times move on and 20.04 is almost out of standart support
• We require 22.04 to have Qt6 for kiwix-desktop 2.5.0
• There is no fondamental reason to keep supporting 20.04

[kelson42]

@rgaudin Wanted to start to move CI to Ubuntu 22.04, starting with libzim. I was kind of optimistic that compiling libzim on Ubuntu 22.04 with all the deps on 20.04 might work. Actually is does not fully work, see my PR and related CI run.

Therefore, it is quite clear with need to move deps and base_deps to be compile on Ubuntu 22.04 as well. We could implement #823 straight away, but this would be a kind of a big bang and probably many CI would be broken. This is not really something I'm willing to do. We should be able to have GNU/Linux (base_)deps at the same time for Ubuntu 20.04 and Ubuntu 22.04.

In any case we have here as well to sort-out the problem temporary solved with #811

[kelson42]

#823 has done most of the job, but we still fail to compile x86-64_musl jobs on Ubuntu (Jammy) 22.04. This is still something left and this is not trivial, therefore assigning issue to @veloman-yunkan.