Implement MSC4169: backwards-compatible redaction sending for rooms < v11 using the /send endpoint

[SpiritCroc]

Implement MSC4169

While there is a dedicated API endpoint for redactions, being able to send redactions using the normal send endpoint is useful when using MSC4140 for sending delayed redactions to replicate expiring messages. Currently this would only work on rooms >= v11 but fail with an internal server error on older room versions when setting the redacts field in the content, since older rooms would require that field to be outside of content. We can address this by copying it over if necessary.

Relevant spec at https://spec.matrix.org/v1.8/rooms/v11/#moving-the-redacts-property-of-mroomredaction-events-to-a-content-property

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[tulir]

I just remembered I made matrix-org/matrix-spec-proposals#4169 which is basically this, so we should add a /versions flag ("com.beeper.msc4169": true) and maybe an experimental_features config option (msc4169_enabled). I'll also update the MSC to remove mass redaction stuff, as matrix-org/matrix-spec-proposals#4343 makes that unnecessary

Test failures here look like flakes?