matrix: set forgotten_room_retention_period to 7d

This commit sets the value for the synapse config option
`forgotten_room_retention_period` to 7 days. This was previously unset,
meaning rooms that had no more local users were never purged from the database.

The new value makes sure that 7 days after the last local user left a
room, it will be permanently deleted from the database.

https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html?highlight=forgotten_room_retention_period#forgotten_room_retention_period

hosts/nachtigall/apps/matrix/synapse.nix

@@ -188,6 +188,7 @@ in
         per_second = 0.17;
       };
       redaction_retention_period = "7d";
+      forgotten_room_retention_period = "7d";
       redis.enabled = false;
       registration_requires_token = false;
       registrations_require_3pid = [ "email" ];