Switch Docker images to Ubuntu 22.04

Even though Ubuntu 20.04 is still supported for about one year, going to Ubuntu 22.04 has some advantages such as providing newer software packages: Guile 3.0.7 is a more relevant testing target than the older Guile 3.0.1 in Ubuntu 20.04, and Texinfo 6.8 allows to drop quite some compatibility code for Texinfo 6.7 from Documentation/lilypond.init.

The only downside is that, once we require some of the versions found in Ubuntu 22.04 as a minimum, LilyPond won't build anymore on 20.04. This is probably fine for master, given that Ubuntu 24.04 was just released, so we will support the current and previous LTS versions.