Reworked Gitlab-CI

[pearzt]

With this PR, I propose a new, revised Gitlab-CI. These are the key changes:

• Environment is managed using Spack
• Designed to run on Lichtenberg's RHEL9 nodes
• Extended matrix of tested GCC and LLVM versions
• Improved CI runtimes, as dependencies (cxxopts, etc.) are managed by Spack and do not need to be rebuilt. Currently, a CI run takes around 6 minutes
• Removed some CI jobs (stripped builds, mostly)
• PGIS is no longer tested in the CI, as it is deprecated anyway

[TimHeldmann]

I going through this PR together with @pearzt, I think it looks good to me.

There are two things that remain after this PR is merged:
One thing that still irks me, but should not be part of this PR, is how many different testing scripts we now have.
In the future I hope we could get some of these scripts either unified, or ported to ctest (or whatever we come up with).

Secondly, it seems there have been problems with compiling if SPD_LOG tries to compile with exceptions.
I have no idea why this would occur. I think this needs some further investigation.