committers-guide: Update current compiler verisons

[jhb]

Summary

• Restructure some of the text to avoid hardcoding compiler versions when possible.

• Explicitly note the naming pattern for llvm toolchain packages.

[jhb]

I'm not sure that the committer's guide is a great place to list compiler versions as it goes stale too easy. I've thought about adding a wiki page with a matrix of compiler versions and branches, but that's likely to be unmaintained as well, hence why I've treated the CI jobs as the real source of truth.

[emaste]

documentation/content/en/articles/committers-guide/_index.adoc

3228

IMO both "should" and "ideally" indicate this isn't a hard requirement and we can leave one out