Declare support for Matrix 1.6

[S7evinK]

Complete changelog v1.6

Client-Server API

Backwards Compatible Changes

• Add information on standard error responses for unknown endpoints/methods, as per MSC3743.
  • Tested within Complement and Dendrite is already passing
• Add /rooms/<roomID>/timestamp_to_event endpoint, as per MSC3030.

Spec Clarifications

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

Server-Server API

Breaking Changes

• Remove keyId from the server /keys endpoints, as per MSC3938.

Backwards Compatible Changes

• Add /timestamp_to_event/<roomID> endpoint, as per MSC3030.
• Extend /_matrix/federation/v2/send_join to allow omitting membership events, per MSC3706.
• Note that /_matrix/federation/v2/send_join should include heroes for nameless rooms, even when allowed to omit membership events, per MSC3943.

Room Versions

Backwards Compatible Changes

• Update the default room version to 10, as per MSC3904.