r0.6 support: /rooms/:roomId/aliases endpoint

[MSC2432](matrix-org/matrix-spec-proposals#2432) added this endpoint originally but it has since been included in the spec for nearly a year. 

We can't reasonably remove the unstable endpoint as client implementations still rely on it, and those clients cannot rely on `r0.6.x` appearing in the `/versions` to detect if this stable endpoint exists. It's proposed to introduce a transition period for implementations which may be stuck on the old endpoint of 2 months (minimum). After that point, Synapse can (and should) remove the unstable endpoint without warning.

The exact timelines are to be determined by the synapse team, however.

This is progress towards #8334

synapse/rest/client/v1/room.py

@@ -892,7 +892,7 @@ class RoomAliasListServlet(RestServlet):
             r"^/_matrix/client/unstable/org\.matrix\.msc2432"
             r"/rooms/(?P<room_id>[^/]*)/aliases"
         ),
-    ]
+    ] + list(client_patterns("/rooms/(?P<room_id>[^/]*)/aliases$"))
 
     def __init__(self, hs: "synapse.server.HomeServer"):
         super().__init__()

tests/rest/client/v1/test_rooms.py

@@ -1799,7 +1799,7 @@ def _get_aliases(self, access_token: str, expected_code: int = 200) -> JsonDict:
         """Calls the endpoint under test. returns the json response object."""
         channel = self.make_request(
             "GET",
-            "/_matrix/client/unstable/org.matrix.msc2432/rooms/%s/aliases"
+            "/_matrix/client/r0/rooms/%s/aliases"
             % (self.room_id,),
             access_token=access_token,
         )