Update default room version to 11 in line with the specification.

roles/custom/matrix-synapse/defaults/main.yml

@@ -1562,7 +1562,7 @@ matrix_synapse_room_list_publication_rules:
     room_id: "*"
     action: allow
 
-matrix_synapse_default_room_version: "10"
+matrix_synapse_default_room_version: "11"
 
 # Controls whether leaving a room will automatically forget it.
 # The upstream default is `false`, but we try to make Synapse less wasteful of resources, so we do things differently.