Declare support for Matrix 1.6

[clokep]

See https://matrix.org/blog/2023/02/14/matrix-v-1-6-release

Client-Server API

• Add information on standard error responses for unknown endpoints/methods, as per MSC3743.
  • See Implement MSC3743 (Standardized error response for unknown endpoints) #14209 and Synapse returns improper error codes for unknown endpoints #14599
• Add /rooms//timestamp_to_event endpoint, as per MSC3030.
  • See Move MSC3030 /timestamp_to_event endpoint to stable v1 location #14471
• Define hkdf-hmac-sha256.v2 MAC method for SAS verification, as per MSC3783.

Server-Server API

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

Application Service API

• Add information on standard error responses for unknown endpoints/methods, as per MSC3743.
  • We would only care about this for calling out to AS, but I don't think we call any potentially unknown endpoints so maybe nothing to do? Could future proof by using the same code we do for federation though.

Identity Service API

• Add information on standard error responses for unknown endpoints/methods, as per MSC3743.
  • We would only care about this for calling out to IS, but I don't think we call any potentially unknown endpoints so maybe nothing to do? Could future proof by using the same code we do for federation though.

Push Gateway API

• Add information on standard error responses for unknown endpoints/methods, as per MSC3743.
  • We would only care about this for calling out to PG, but I don't think we call any potentially unknown endpoints so maybe nothing to do? Could future proof by using the same code we do for federation though.

Room Versions

• Update the default room version to 10, as per MSC3904.
  • See Change default room version to 10. Implements MSC3904 #14111

[clokep]

I've attempted to add some notes about what I think is done, but likely could use double checking. See #14486 for a previous iteration.

[clokep]

Define hkdf-hmac-sha256.v2 MAC method for SAS verification

The legacy. method (hkdf-hmac-sha256) doesn't appear in the Synapse repo at all, so I'm going to assume there's nothing to do here.

[clokep]

Application Service API / Identity Service API / Push Gateway API

• Add information on standard error responses for unknown endpoints/methods

#15317 ended up using this for appservices, will need to split this into three tasks. I think.