Document Rosetta cask support policy

- explain in `docs/Acceptable-Casks.md` how
  `requires_rosetta` casks remain acceptable while Rosetta 2
  is still available, and when that is expected to change
- record the expected macOS 27 and macOS 28 milestones so
  future deprecate/disable decisions have documented timing
- clarify in `docs/Support-Tiers.md` that Rosetta affects
  package policy rather than the support tier of the host