Declare support for Matrix v1.8

[clokep]

See the changelog (note that I stripped out spec clarifications).

This also requires us to finish support for Matrix v1.7: see #15661.

Client-Server API

Backwards Compatible Changes

• Require callers to be joined to the room to report its events, as per MSC2249.
  • See Prevent clients from reporting nonexistent events. #13779.

Server-Server API

Backwards Compatible Changes

• Add matrix-fed SRV lookup steps to server discovery, as per MSC4040.
  • See Add MSC4040 matrix-fed service lookups #16137

Room Versions

Backwards Compatible Changes

• Add room version 11 as per MSC3820.
  • Move redacts from top level to content on m.room.redaction events in room version 11, as per MSC2174.
  • Remove creator from m.room.creator events in room version 11, as per MSC2175.
  • Remove remaining usage of origin from events in room version 11, as per MSC3989.
  • Update the redaction rules in room version 11, as per MSC2176 and MSC3821.
  • See Support room version 11 #15912.

Appendices

Backwards Compatible Changes

• Allow + in Matrix IDs, as per MSC4009.
  • See Update allowed characters for Matrix IDs (MSC4009) #15911.