Declare support for Matrix 1.8

*This issue was originally created by [**@S7evinK**](https://github.com/S7evinK) at .*

[Complete changelog v1.8](https://github.com/matrix-org/matrix-spec/blob/e40d9ca18676fc774b39f30b44998d7f0118596a/content/changelog/v1.8.md)

### Client-Server API

**Backwards Compatible Changes**

- [x] Require callers to be joined to the room to report its events, as per [MSC2249](https://github.com/matrix-org/matrix-spec-proposals/pull/2249).
- No `/report` endpoint at all (see https://github.com/matrix-org/dendrite/issues/3216)

**Spec Clarifications**

- [ ] Allow `null` in `room_types` in `POST /publicRooms` endpoints schemas. ([#1564](https://github.com/matrix-org/matrix-spec/issues/1564))
- No filtering atm (see https://github.com/matrix-org/dendrite/issues/3222)

### Server-Server API

**Deprecations**
- [x] Deprecate `matrix` SRV lookup steps during server discovery, as per [MSC4040](https://github.com/matrix-org/matrix-spec-proposals/pull/4040).
- https://github.com/matrix-org/gomatrixserverlib/pull/411

**Backwards Compatible Changes**
- [x] Add `matrix-fed` SRV lookup steps to server discovery, as per [MSC4040](https://github.com/matrix-org/matrix-spec-proposals/pull/4040).
- https://github.com/matrix-org/gomatrixserverlib/pull/411

**Spec Clarifications**
- [x] Document why `/state_ids` can respond with a 404. ([#1521](https://github.com/matrix-org/matrix-spec/issues/1521))
- We already 404 if we can't find the event

### Room Versions

**Backwards Compatible Changes**

- [x] Add room version 11 as per [MSC3820](https://github.com/matrix-org/matrix-spec-proposals/pull/3820).
- Move `redacts` from top level to `content` on `m.room.redaction` events in room version 11, as per [MSC2174](https://github.com/matrix-org/matrix-spec-proposals/pull/2174).
- Remove `creator` from `m.room.creator` events in room version 11, as per [MSC2175](https://github.com/matrix-org/matrix-spec-proposals/pull/2175).
- Remove remaining usage of `origin` from events in room version 11, as per [MSC3989](https://github.com/matrix-org/matrix-spec-proposals/pull/3989).
- Update the redaction rules in room version 11, as per [MSC2176](https://github.com/matrix-org/matrix-spec-proposals/pull/2176) and [MSC3821](https://github.com/matrix-org/matrix-spec-proposals/pull/3821).

### Appendices

**Backwards Compatible Changes**

- [x] Allow `+` in Matrix IDs, as per [MSC4009](https://github.com/matrix-org/matrix-spec-proposals/pull/4009).

Start by reviewing the unchecked Matrix 1.8 item for allowing null in room_types in the POST /publicRooms endpoint schemas, along with linked issue #3222. Check the relevant public-room schema and endpoint behavior; done means the remaining Matrix 1.8 requirement is implemented and validated without changing the already checked items.

Written by the indexing model from the issue text.