[ci] re-organize CI into a graph of reusable workflows #7463

Contributes to #7462

Proposes wrapping all required CI in a single workflow that calls all others. See #7462 for a list of the benefits, but in short... this allows finer-grained control of CI, should make end-to-end CI time for PRs shorter, and will help with removing conda from Python CI jobs (#7355).

Notes for Reviewers

Branch Protections will need an update

If this is approved, I'll update branch protections to only require the new all-required-pr-jobs job for PRs to be mergeable. At that moment, all other open PRs will need to pull in latest main, but I think that's ok.