Promote account suspension to stable

[turt2live]

MSC: matrix-org/matrix-spec-proposals#3823

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

🚨 DO NOT MERGE UNTIL MSC3823 FINISHES FCP SUCCESSFULLY

As of writing, the MSC only just entered FCP, which must conclude successfully before this can be merged. Early review is appreciated to facilitate the fastest possible process though.