matrix: Update list of protected rooms

roles/matrix/templates/mjolnir.yaml.j2

@@ -79,6 +79,7 @@ automaticallyRedactForReasons:
 # A list of rooms to protect (matrix.to URLs)
 protectedRooms:
   - "https://matrix.to/#/#archlinux:{{ matrix_server_name }}"
+  - "https://matrix.to/#/#offtopic:{{ matrix_server_name }}"
 
 # Set this option to true to protect every room the bot is joined to. Note that
 # this effectively makes the protectedRooms and associated commands useless because