implement MSC2174: Move the `redacts` key to a sane place

[richvdh]

It would sure be nice to implement MSC2174 - maybe even in time to get it into room v6.

[tulir]

It would be very not nice to have to migrate redaction event format twice

[anoadragon453]

This is now slated for room v7+.

[richvdh]

It would be very not nice to have to migrate redaction event format twice

I'm not quite sure what you mean by this. If you're against the principle of MSC2174, then those comments belong on the MSC.

[tulir]

This step should be skipped and we should go straight for mass redactions.

[richvdh]

ah right. yes, maybe. Mass redactions are MSC2244, ftr.

[turt2live]

Required for https://github.com/matrix-org/matrix-doc/issues/3097

[clokep]

Note that #8984 is somewhat related to this as MSC2176 updates the redaction rules based on MSC2174. There's some confusing overlap there.