Document minimum HTTP versions for Federation and Client-Server API

[turt2live]

#696 but for HTTP(S).

The Federation API implies HTTP/1.1 through https://spec.matrix.org/v1.8/server-server-api/#request-authentication

[richvdh]

In practice, today, I'd consider HTTP/1.1 as a required baseline, and HTTP/2 or HTTP/3 as optional improvements.