matrix.org advertises `org.matrix.msc2965.authentication` in its well-known

https://matrix.org/.well-known/matrix/client includes an entry for `org.matrix.msc2965.authentication`. As the name suggests, this was used in a draft version of MSC2965, but was later superseded by a separate endpoint. We should remove the unspecced property.

For future readers: clients should be using the stable `GET /_matrix/client/v1/auth_metadata` endpoint from the OAuth 2.0 API [server metadata discovery](https://spec.matrix.org/v1.15/client-server-api/#server-metadata-discovery).