Declare support for Matrix 1.5

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

[Complete changelog v1.5](https://github.com/matrix-org/matrix-spec/blob/e40d9ca18676fc774b39f30b44998d7f0118596a/content/changelog/v1.5.md)

### Client-Server API

Backwards Compatible Changes

- [ ] Add `m.reference` relations, as per [MSC3267](https://github.com/matrix-org/matrix-spec-proposals/pull/3267).
- Only missing aggregation, if anything?

Spec Clarifications
(NOTE: non-actionable things removed, as well as things we don't have implemented at all yet, e.g. refresh tokens clarification would be fixed when implementing refresh tokens)

- [x] Define a 400 response from `/_matrix/client/v3/directory/rooms/{roomAlias}`. ([#1286](https://github.com/matrix-org/matrix-spec/issues/1286))
- We already return 400, but not the correct `errcode`

### Application Service API

Spec Clarifications

- [ ] Clarify that application services can only register an interest in local users, as per [MSC3905](https://github.com/matrix-org/matrix-spec-proposals/issues/3905). ([#1305](https://github.com/matrix-org/matrix-spec/issues/1305))
- From the looks of it, we just compile the provided regex, but don't check if the user is local

Start by reviewing the Matrix 1.5 changelog and the linked MSC3267 and MSC3905 proposals. Check the Client-Server room-alias endpoint and application-service user-interest registration, including the noted error-code and local-user validation gaps. Done means the remaining unchecked Matrix 1.5 requirements are implemented and verified against the relevant API behavior.

Written by the indexing model from the issue text.