Declare support for Matrix 1.2

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

A list of things to actually announce support for 1.2 (while we already do)
While this is about getting actual 1.2 support, it make sense to check https://spec.matrix.org/latest/

[Complete changelog v1.2](https://github.com/matrix-org/matrix-spec/blob/e40d9ca18676fc774b39f30b44998d7f0118596a/content/changelog/v1.2.md)

### Client-Server API

Breaking Changes

- [x] The `prev_content` field is now returned inside the `unsigned` property of events, rather than at the top level, as per [MSC3442](https://github.com/matrix-org/matrix-doc/pull/3442).
- As per the MSC, Dendrite already returns `prev_content` in `unsigned`. The only struct using `prev_content` is [here](https://github.com/matrix-org/dendrite/blob/3d02c8103152fbb9a5401a091ef0f681a32bfeee/clientapi/routing/state.go#L35), which is unused.
- [x] The `aliases` property from the chunks returned by `/publicRooms`, as per [MSC2432](https://github.com/matrix-org/matrix-doc/pull/2432).
- Needs to be removed in GMSL

New Endpoints

- [x] Add the Space Hierarchy API (`GET /_matrix/client/v1/rooms/{roomId}/hierarchy`) as per [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946).
- [ ] Add `/_matrix/client/v1/register/m.login.registration_token/validity` as per [MSC3231](https://github.com/matrix-org/matrix-doc/pull/3231).
- Missing endpoint, admin API to create tokens etc. already exists

Backwards Compatible Changes

- [x] Extend `/_matrix/client/r0/login` to accept a `m.login.appservice`, as per [MSC2778](https://github.com/matrix-org/matrix-doc/pull/2778).
- See https://github.com/matrix-org/dendrite/pull/3078
- [x] Add support for `restricted` rooms as per [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083), [MSC3289](https://github.com/matrix-org/matrix-doc/pull/3289), and [MSC3375](https://github.com/matrix-org/matrix-doc/pull/3375).
- [x] Add `is_guest` to `/account/whoami` as per [MSC3069](https://github.com/matrix-org/matrix-doc/pull/3069).
- [x] Expand guest access to sending any room event and state event as per [MSC3419](https://github.com/matrix-org/matrix-doc/pull/3419).
- [x] Add Spaces and room types as per [MSC1772](https://github.com/matrix-org/matrix-doc/pull/1772) and [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946).
- I think we do?
- [ ] Add new `m.set_displayname`, `m.set_avatar_url`, and `m.3pid_changes` capabilities as per [MSC3283](https://github.com/matrix-org/matrix-doc/pull/3283).
- [x] Add support for fallback keys (optional keys used once one-time keys run out), as per [MSC2732](https://github.com/matrix-org/matrix-doc/pull/2732).
- [ ] Add token-authenticated registration support as per [MSC3231](https://github.com/matrix-org/matrix-doc/pull/3231).

### Server-Server API

New Endpoints

- [x] Add the Space Hierarchy API (`GET /_matrix/federation/v1/hierarchy/{roomId}`) as per [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946).

Backwards Compatible Changes

- [x] Add support for `restricted` rooms as per [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083), [MSC3289](https://github.com/matrix-org/matrix-doc/pull/3289), and [MSC3375](https://github.com/matrix-org/matrix-doc/pull/3375).

### Room Versions

Backwards Compatible Changes

- [x] Add Room Version 8 as per [MSC3289](https://github.com/matrix-org/matrix-doc/pull/3289). ([#3387](https://github.com/matrix-org/matrix-doc/issues/3387))
- [x] Add Room Version 9 as per [MSC3375](https://github.com/matrix-org/matrix-doc/pull/3375). ([#3387](https://github.com/matrix-org/matrix-doc/issues/3387))