Phasing out EOL distributives

> @develop7 you're right. I removed all unrelated CI changes from this PR.
> I commented out the buster stuff out of frustration because I didn't understand where the failing jobs are coming from.
> (It was because I was missing the exclusions for 9.12 which doesn't have buster/bullseye variants.)
>
> But in general are you in favor of both removing buster from CI as well as removing all buster dockerfiles from the repo in separate PR?
> My arguments for removal: 1. don't waste CI resources building something that's past the EOL. 2. make the repo easier to navigate 3. if anyone wants those buster dockerfiles they can find them in git history.
>

_Originally posted by @jhrcek in https://github.com/haskell/docker-haskell/issues/151#issuecomment-2780918778_