matrix: Increase size of get_users_in_room cache

This cache was seeing constant evictions at its former size (70k).

With a size of 200k it now stabilizes at ~122k entries with a
significant drop in server load.

roles/matrix/templates/homeserver.yaml.j2

@@ -715,6 +715,7 @@ caches:
   #
   per_cache_factors:
     #get_users_who_share_room_with_user: 2.0
+    get_users_in_room: 2.0
 
   # Controls how long an entry can be in a cache without having been
   # accessed before being evicted. Defaults to None, which means