CI: drop 3.6 & ubuntu switch

[tylerjereddy]

• the Github Actions infrastructure is progressively phasing in a newer base image of Ubuntu: https://github.blog/changelog/2022-11-09-github-actions-ubuntu-latest-workflows-will-use-ubuntu-22-04/

• that means that we are somewhat randomly going to see images that lack Python 3.6 from the build cache sometimes per: https://github.com/actions/setup-python/issues/355#issuecomment-1335042510

• instead of pinning to an old version of Ubuntu in GHA for 3.6 support, let's just drop 3.6 from the testing matrix per: https://github.com/darshan-hpc/darshan/issues/510#issuecomment-1340023264 (it has been EOL for 1 year)

• there's also no reason to retain the special Python 3.10 handling where we use Ubuntu 22.04 for that case, for two reasons:

1) 22.04 is being rolled out as the new default anyway 2) gh-851 with the Python garbage collection issues in 3.10 was resolved by using contexts, so special treatment not justified anymore

[shanedsnyder]

To fully remove on from 3.6, there are still some other tasks, correct? Off the top of my head, I guess pyproject.toml needs to be updated so cibuildwheel doesn't build wheels for it, but probably other things, too, I'm guessing.

Are those out of scope here? Or is it worth ripping the bandaid off?

[tylerjereddy]

I was just going to remove the CI support to start, I don't really care if we continue here though, but you'll end up with randomly failing CI for awhile if you wait

[shanedsnyder]

Maybe I'll just merge this in, and can start up a more general PR shortly, just to avoid any distractions on any other PRs in the mean time.