Update Synapse default room version (6 -> 9)

From the [Synapse 1.43.0 release highlights](https://matrix.org/blog/2021/09/21/synapse-1-43-0-released):
> Asks clients to prefer [room version 9](https://github.com/matrix-org/matrix-doc/pull/3375) when creating restricted rooms ([#10772](https://github.com/matrix-org/synapse/issues/10772)), via the API defined in [MSC3244: room version capabilities](https://github.com/matrix-org/matrix-doc/pull/3244).

roles/matrix-synapse/defaults/main.yml

@@ -561,7 +561,7 @@ matrix_synapse_room_list_publication_rules:
     room_id: "*"
     action: allow
 
-matrix_synapse_default_room_version: "6"
+matrix_synapse_default_room_version: "9"
 
 # Controls the Synapse `spam_checker` setting.
 #