Changed the workflow to be able to support installation of older versions of clang

[cpjulia]

This PR changes the CI workflow for correcting the error that occurs when tests begin (shown in the screenshot below).

GitHub's ubuntu-latest runner switched to a later Ubuntu version that doesn't support clang 11 and 12 in its default repositories, and these versions are included in our matrix. As can be seen in the screenshot, the package list shows entries the prefix noble which is for Ubuntu 24.04, not jammy jellyfish anymore as in 22.04.
So we have to go to llvm-toolchain repository for the correct Ubuntu version codename to allow apt to install the versions needed. It's ends up being a mix of both distributions, but so far the only solution found.
EDIT: trying now with focal fossa because there's no repository for clang-12 in jammy either

[neunhoef]

I think a different approach is needed here. It will be hard to install all these old clang versions, but they are very old. After all, we are at clang-21 now. So we should change the versions of clang for which we test. For 3.12, we compile with clang-19, so this should be in the mix. Maybe we stick to the versions of clang which are in popular Ubuntu releases? It seems that Ubuntu 24.04 LTS has clang versions 14 to 20 and the default is 18. And it has gcc-9 to gcc-14 with 13 being the default. Let's try to use only that Ubuntu and then as many compilers as work reasonably from this list.