Refactor actions to make fewer status checks

[zzak]

This flattens our workflow graph from 8 checks to just 3.

By compacting the lint jobs into a single job inside a single workflow, we maybe lose some readability -- but since the cost to maintain those files is already really low, they are rarely changed, versus the number of checks on each PR makes for a worse experience IMO.

This also means that all of those separate checks can be found on a single page (e.g. here), for better or worse.. navigating the actions UI across multiple workflows and jobs is already strenuous. There is an argument that if one of these steps fails, debugging it there may be difficult, but I've tried to order them in terms of priority of "if this step fails it should fail first".

If any of the Lint steps fails, figuring out which one and why may not be as straight-forward, that maybe the only downside here. There are plans to move this workflow entirely to Buildkite at some point, probably after a refactor (rails/buildkite-config#62, where they can go back to being separate "jobs" in that context and all of the important stuff is in a single place.

So this is more of an intermediary step to improve the PR page and hopefully make reviewing them and deciding which checks failed matters, more efficient.