Declare support for Matrix 1.2

[S7evinK]

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

Client-Server API

Breaking Changes

• The prev_content field is now returned inside the unsigned property of events, rather than at the top level, as per MSC3442.
  • As per the MSC, Dendrite already returns prev_content in unsigned. The only struct using prev_content is here, which is unused.
• The aliases property from the chunks returned by /publicRooms, as per MSC2432.
  • Needs to be removed in GMSL

New Endpoints

• Add the Space Hierarchy API (GET /_matrix/client/v1/rooms/{roomId}/hierarchy) as per MSC2946.
• Add /_matrix/client/v1/register/m.login.registration_token/validity as per MSC3231.
  • Missing endpoint, admin API to create tokens etc. already exists

Backwards Compatible Changes

• Extend /_matrix/client/r0/login to accept a m.login.appservice, as per MSC2778.
  • See Appservice Login (2nd attempt) #3078
• Add support for restricted rooms as per MSC3083, MSC3289, and MSC3375.
• Add is_guest to /account/whoami as per MSC3069.
• Expand guest access to sending any room event and state event as per MSC3419.
• Add Spaces and room types as per MSC1772 and MSC2946.
  • I think we do?
• Add new m.set_displayname, m.set_avatar_url, and m.3pid_changes capabilities as per MSC3283.
• Add support for fallback keys (optional keys used once one-time keys run out), as per MSC2732.
• Add token-authenticated registration support as per MSC3231.

Server-Server API

New Endpoints

• Add the Space Hierarchy API (GET /_matrix/federation/v1/hierarchy/{roomId}) as per MSC2946.

Backwards Compatible Changes

• Add support for restricted rooms as per MSC3083, MSC3289, and MSC3375.

Room Versions

Backwards Compatible Changes

• Add Room Version 8 as per MSC3289. (#3387)
• Add Room Version 9 as per MSC3375. (#3387)