Room IDs being splittable into localpart and servername does not have
much inherent value and there are proposals like MSC4051¹ that propose
changing the format. Relaxing the rules makes Ruma forwards-compatible
with those proposals. The server_name accessor is kept because it is
used by at least one downstream, but is updated to return an `Option`.
¹ https://github.com/matrix-org/matrix-spec-proposals/pull/4051
