Fix space hierarchy endpoint to match MSC2946 (#11667)

Fixes minor discrepancies between the /hierarchy endpoint described
in MSC2946 and the implementation.

Note that the changes impact the stable and unstable /hierarchy and
unstable /spaces endpoints for both client and federation APIs.

changelog.d/11667.bugfix

@@ -0,0 +1 @@
+Fix `/_matrix/client/v1/room/{roomId}/hierarchy` endpoint returning incorrect fields which have been present since Synapse 1.49.0.

synapse/handlers/room_summary.py

@@ -209,7 +209,7 @@ async def get_space_summary(
                         # Before returning to the client, remove the allowed_room_ids
                         # and allowed_spaces keys.
                         room.pop("allowed_room_ids", None)
-                        room.pop("allowed_spaces", None)
+                        room.pop("allowed_spaces", None)  # historical
 
                         rooms_result.append(room)
                         events.extend(room_entry.children_state_events)
@@ -988,12 +988,14 @@ async def _build_room_entry(self, room_id: str, for_federation: bool) -> JsonDic
             "canonical_alias": stats["canonical_alias"],
             "num_joined_members": stats["joined_members"],
             "avatar_url": stats["avatar"],
+            # plural join_rules is a documentation error but kept for historical
+            # purposes. Should match /publicRooms.
             "join_rules": stats["join_rules"],
+            "join_rule": stats["join_rules"],
             "world_readable": (
                 stats["history_visibility"] == HistoryVisibility.WORLD_READABLE
             ),
             "guest_can_join": stats["guest_access"] == "can_join",
-            "creation_ts": create_event.origin_server_ts,
             "room_type": create_event.content.get(EventContentFields.ROOM_TYPE),
         }