move to alma8 by default; add alma9 & enable choice through DEFAULT_LINUX_VERSION

[h-vetinari]

Alternative to #6548

Closes #6283
Closes #6629

We can do this because all the required infrastructure is now in place (c.f. conda-forge/conda-forge.github.io#1941); the last missing piece was to verify how entries of yum_requirements.txt match between centos7 & alma8 - the following overview shows that there's a high degree of compatibility (and many things can be replaced by our own build anyway). The handful of differences can be fixed on the affected feedstocks.

Should wait for conda-forge/docker-images#291 & conda-forge/cdt-builds#71

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

[h-vetinari]

For the reasoning behind 24b7e66, see discussion here. The preceding commit fails if a feedstock would override c_stdlib_version, see also this branch where I've tested this for an affected feedstock.

24b7e66 would thus be necessary to make the docker image (sufficiently) independent from c_stdlib_version, so that they can be set independently (if indeed this is something we desire to enable; one reason why is that the image version doesn't have to affect the sysroot-baseline, in particular if it's only for being able to load a build tool or test dependency).