Stabilize MSC3266, support stable room_summary endpoint

[dasha-uwu]

MSC3266 is merged in v1.15, let's stabilize it as part of #18731

1. Add support for the stable /_matrix/client/v1/room_summary/ endpoint, keeping both unstable endpoints for compat
2. Remove the experimental msc3266_enabled flag

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)

[erikjohnston]

Thanks!

[MadLittleMods]

@erikjohnston Please check off #18731 as necessary to what this PR solves (include links to this PR on the points it solves)