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.

[jplehr]

I personally disagree with removing PGIS from the build. I understand that it is deprecated. The reason I disagree is because it is the one (only one?) tool that acts as a full end-to-end test for the library and package components.

I simply do not trust the rest of our testing enough.

If this is what we want to do, I won't get in the way, but I want to raise my concern.

[jplehr]

Can / should we no-matter-what put names in ' here?
My inner monk does not like that it is done sometimes and sometimes not.

[jplehr]

Do we actually still need the stages or can we remove them as we now simply use needs to define the DAG?

Helps with clear visualization?