Add `MSC3820opt2` as a known room version (#15678)

changelog.d/15678.misc

@@ -0,0 +1 @@
+Implement "option 2" for [MSC3820](https://github.com/matrix-org/matrix-spec-proposals/pull/3820): Room version 11.

synapse/api/room_versions.py

@@ -528,6 +528,7 @@ KNOWN_ROOM_VERSIONS: Dict[str, RoomVersion] = {
         RoomVersions.V10,
         RoomVersions.MSC2716v4,
         RoomVersions.MSC3989,
+        RoomVersions.MSC3820opt2,
     )
 }