This updates all images in the CI to Ubuntu 22. It also updates
corresponding documentation. Remove python(2) from the image, as it is
not needed. Update documentation.

It seems that the gcc coming in Ubuntu 22 does not like
stack-protector-strong, so remove it as well.

Since we compile Flexric, which does not support Ubuntu 22's default
gcc-11 (it hits a compile bug), switch to gcc-12 manually.

Since we update the compiler, we also need to be careful to use the right
address sanitizer version (which is bundled with the compiler version).
To avoid a reinstallation of the wrong asan version in target images, we
modify Ubuntu 22 targets to copy libasan from the base image. This is necessary
since we use update-alternative in the base image, but we don't want to
install compilers in the target images to use update-alternatives to
install the right asan version... This should be undone when switching
to Ubuntu 24 in the future, as it will have a newer default gcc version,
suc...