Unprefix room_version & encryption from hierarchy API (#19576)

These seem to work properly in the hierarchy API so I figured I'd
unprefix them and remove the flag requirement. This should [better align
with the
spec](https://spec.matrix.org/latest/client-server-api/#get_matrixclientv1roomsroomidhierarchy_response-200_spacehierarchyroomschunk)
and Matrix v1.15 (via
[MSC3266](https://github.com/matrix-org/matrix-spec-proposals/pull/3266)),
which says these properties are available and named like this.

Related to #18731.

This doesn't affect the stability or experimental flags of the room
summary API, just the hierarchy API.

changelog.d/19576.feature

@@ -0,0 +1 @@
+Stabilize `room_version` and `encryption` fields in the space/room `/hierarchy` API (part of [MSC3266](https://github.com/matrix-org/matrix-spec-proposals/pull/3266)).

synapse/handlers/room_summary.py

@@ -128,7 +128,6 @@ class RoomSummaryHandler:
             name="get_room_hierarchy",
             server_name=self.server_name,
         )
-        self._msc3266_enabled = hs.config.experimental.msc3266_enabled
 
     async def get_room_hierarchy(
         self,
@@ -791,6 +790,7 @@ class RoomSummaryHandler:
 
         entry: JsonDict = {
             "room_id": stats.room_id,
+            "room_version": stats.version,
             "name": stats.name,
             "topic": stats.topic,
             "canonical_alias": stats.canonical_alias,
@@ -802,12 +802,9 @@ class RoomSummaryHandler:
             ),
             "guest_can_join": stats.guest_access == "can_join",
             "room_type": stats.room_type,
+            "encryption": stats.encryption,
         }
 
-        if self._msc3266_enabled:
-            entry["im.nheko.summary.version"] = stats.version
-            entry["im.nheko.summary.encryption"] = stats.encryption
-
         # Federation requests need to provide additional information so the
         # requested server is able to filter the response appropriately.
         if for_federation: