Declare support for Matrix 1.6

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

[Complete changelog v1.6](https://github.com/matrix-org/matrix-spec/blob/e40d9ca18676fc774b39f30b44998d7f0118596a/content/changelog/v1.6.md)

### Client-Server API

Backwards Compatible Changes

- [x] Add information on standard error responses for unknown endpoints/methods, as per [MSC3743](https://github.com/matrix-org/matrix-spec-proposals/pull/3743).
- Tested within Complement and Dendrite is already passing
- [ ] Add `/rooms//timestamp_to_event` endpoint, as per [MSC3030](https://github.com/matrix-org/matrix-spec-proposals/pull/3030).

Spec Clarifications

- [ ] Clarify the power levels integer range. ([#1169](https://github.com/matrix-org/matrix-spec/issues/1169), [#1355](https://github.com/matrix-org/matrix-spec/issues/1355))
- I don't think we validate the power levels integer ranges
- [x] Clarify that `/context` always returns `event` even if `limit` is zero. ([#1239](https://github.com/matrix-org/matrix-spec/issues/1239))
- Currently our `limit` filter is passed on to `events_before` and `events_after`, need to `filter.Limit/2`
- [x] Clarify what fields are required when deleting a pusher ([#1321](https://github.com/matrix-org/matrix-spec/issues/1321))
- I think we already handle this correctly
- [x] Correct the order of the default override pushrules in the spec. ([#1421](https://github.com/matrix-org/matrix-spec/issues/1421))

### Server-Server API

Breaking Changes

- [ ] Remove `keyId` from the server `/keys` endpoints, as per [MSC3938](https://github.com/matrix-org/matrix-spec-proposals/pull/3938).

Backwards Compatible Changes

- [ ] Add `/timestamp_to_event/` endpoint, as per [MSC3030](https://github.com/matrix-org/matrix-spec-proposals/pull/3030).
- [ ] Extend `/_matrix/federation/v2/send_join` to allow omitting membership events, per [MSC3706](https://github.com/matrix-org/matrix-doc/pull/3706).
- [ ] Note that `/_matrix/federation/v2/send_join` should include heroes for nameless rooms, even when allowed to omit membership events, per [MSC3943](https://github.com/matrix-org/matrix-doc/pull/3943).

### Room Versions

Backwards Compatible Changes

- [x] Update the default room version to 10, as per [MSC3904](https://github.com/matrix-org/matrix-spec-proposals/pull/3904).