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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[MadLittleMods]

Suggested change

	Drop support for unstable field names from the long-accepted MSC2732 proposal.
	Drop support for unstable field names from the long-accepted [MSC2732](https://github.com/matrix-org/matrix-spec-proposals/pull/2732) (Olm fallback keys) proposal.

[MadLittleMods]

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

And I guess this branch is just out of date