create alma9-based infrastructure

[h-vetinari]

This is similar to #1941, only that we're way earlier in the process. But like we had alma8 available while our default was still CentOS 6, we are in a situation where alma9-based infrastructure will be beneficial despite not being the default for a long time yet. Especially now with {{ stdlib("c") }} in place, this will be much less of a hassle.

The underlying RHEL 9.0 was released almost 2.5 years ago, and the first projects are starting to require newer glibc's than 2.28 (which itself is >6 years old). An indeed, based on https://mayeut.github.io/manylinux-timeline/ at least¹, over 60% of PyPI downloads for clients on non-EOL python versions already have glibc >=2.34.

The manylinux project will release alma9-based images soon (c.f. pypa/manylinux#1585) and we should likewise be ready for whenever projects start requiring this.

• Glibc 2.34 from alma 9 linux-sysroot-feedstock#80
• Add alma9 images docker-images#290
• Add cdts for alma9; use cdt_name=conda for all distros cdt-builds#71

Footnotes

1. I'd love to have an equivalent for conda-forge data... ↩

[h-vetinari]

Alright, with the merge of conda-forge/conda-forge-pinning-feedstock#6626, we actually leap-frogged #1941 and basically finished the roll-out. Last missing piece is staged recipes: conda-forge/staged-recipes#28326