Remove unstable prefixes for MSC2732: Olm fallback keys

[anoadragon453]

MSC2732 (Olm fallback keys) was accepted in 2022. We shouldn't need to continue supporting the unstable field names.

Split out from #17097.

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Note: this PR was accidentally committed to develop early (2aab171), hence the minimal diff in the PR, which now only introduces changelog entry for the changes.

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)

[MadLittleMods]

It looks like you might have pushed this to develop already -> 2aab171

And I guess this branch is just out of date

[anoadragon453]

Oops! That would explain why the unit tests on #17097 were passing... Likely happened when performing a release after adding this commit to my local develop checkout.