MSC2174: move the redacts property to content

m.room.redaction events currently have an event-level property redacts which gives the event ID of the event being redacted.

The presence of this field at the event level, rather than under the content key, is anomalous. This MSC proposes that, in a future room version, the redacts property be moved under the content key.

For backwards-compatibility with older clients, servers should add a redacts property to the top level of m.room.redaction events in newer room versions when serving such events over the Client-Server API.

For improved compatibility with newer clients, servers should add a redacts property to the content of m.room.redaction events in older room versions when serving such events over the Client-Server API.

Unstable prefix

Implementations should use org.matrix.msc2174 for the room version containing this change until assigned to a specified room version. That room version should use room version 6 as a base.