Accept unprefixed form of MSC3981 recurse parameter (#16842)

Now that the MSC3981 has passed FCP

changelog.d/16842.misc

+Add support for stabilised [MSC3981](https://github.com/matrix-org/matrix-spec-proposals/pull/3981) that adds a `recurse` parameter on the `/relations` API.

synapse/rest/client/relations.py

@@ -71,7 +71,7 @@ class RelationPaginationServlet(RestServlet):
             self._store, request, default_limit=5, default_dir=Direction.BACKWARDS
         )
         if self._support_recurse:
-            recurse = parse_boolean(
+            recurse = parse_boolean(request, "recurse", default=False) or parse_boolean(
                 request, "org.matrix.msc3981.recurse", default=False
             )
         else: