reflect that rooms v3 is a stable room version

synapse/api/constants.py

@@ -119,6 +119,7 @@ KNOWN_ROOM_VERSIONS = {
     RoomVersions.V2,
     RoomVersions.VDH_TEST,
     RoomVersions.STATE_V2_TEST,
+    RoomVersions.V3,
 }
 
 ServerNoticeMsgType = "m.server_notice"

synapse/rest/client/v2_alpha/capabilities.py

@@ -53,6 +53,7 @@ class CapabilitiesRestServlet(RestServlet):
                             "1": "stable",
                             "2": "stable",
                             "state-v2-test": "unstable",
+                            "3": "stable",
                         }
                     },
                     "m.change_password": {