Always treat RETURNING as supported by SQL engines

[AndrewFerr]

Can do this now that SQLite 3.40 supports RETURNING.

This also bumps the minimum suppoted SQLite version according to Synapse's deprecation policy.

Addresses #17577

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)

[reivilibre]

seems good, CI is failing though (maybe a missed usage of supports_returning?)

[AndrewFerr]

This needs the Sytest image to install SQLite >=3.40.

See: matrix-org/sytest#1414