Add some roomserver UTs (#3067)

Adds tests for `QueryRestrictedJoinAllowed`, `IsServerAllowed` and
`PerformRoomUpgrade`. Refactors the `QueryRoomVersionForRoom` method to
accept a string and return a `gmsl.RoomVersion` instead of req/resp
structs.
Adds some more caching for `GetStateEvent`

This should also fix #2912 by ignoring state events belonging to other
users.

clientapi/routing/server_notices.go

@@ -157,7 +157,6 @@ func SendServerNotice(
 			Visibility:                "private",
 			Preset:                    presetPrivateChat,
 			CreationContent:           cc,
-			GuestCanJoin:              false,
 			RoomVersion:               roomVersion,
 			PowerLevelContentOverride: pl,
 		}