move to alma9 by default; 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:

• Add cdts for alma9; use cdt_name=conda for all distros cdt-builds#71
• Add alma9 images docker-images#290 (if we want to add the alma9 images already)

Related (but not necessary here as image version for CUDA 11.8 isn't overridable anyway; unless we do the variant of conda-forge/docker-images#293 that puts the distro version in the tag):

• Provide linux-anvil-cuda on x64 with RHEL 8, distinguish distro baseline in tag docker-images#291

[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 c7f5973, 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.

c7f5973 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).

[h-vetinari]

@jakirkham, you mentioned in the core call just now that all the CUDA feedstocks already set os_version explicitly? In only did a spot check and it looks like this for example:

os_version:
  linux_64: cos7
  linux_aarch64: cos7
  linux_ppc64le: cos7

Following conda-forge/docker-images#291 (using the consolidated names that allow choosing the distro version), this would eventually have to move to

os_version:
  linux_64: cos7
  linux_aarch64: ubi8
  linux_ppc64le: ubi8

or just

os_version:
  linux_64: cos7

because we don't/won't support distro overrides for aarch/ppc. In either case, once we've confirmed that we got os_version set in the relevant feedstocks doing binary repackaging, this PR should be ready. I'll leave out the image renaming stuff for #6687