Change matrix_synapse_forgotten_room_retention_period from null to 28d

As we automatically forget rooms on leave in the playbook this option working at all is probably heavily desired.

Timing is copied from upstream example.

Credit goes to Anoa for making me even check this.

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

@@ -1456,7 +1456,7 @@ matrix_synapse_redaction_retention_period: 7d
 # Controls how long to keep locally forgotten rooms before purging them from the DB.
 # Defaults to `null`, meaning it's disabled.
 # Example value: 28d
-matrix_synapse_forgotten_room_retention_period: ~
+matrix_synapse_forgotten_room_retention_period: 28d
 
 matrix_synapse_user_ips_max_age: 28d