Add changelog entry for the forget_rooms_on_leave default change for Synapse

Related to:

- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2698
- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2700

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

@@ -928,6 +928,10 @@ matrix_synapse_room_list_publication_rules:
 
 matrix_synapse_default_room_version: "10"
 
+# 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.
+matrix_synapse_forget_rooms_on_leave: true
+
 # Controls the Synapse `spam_checker` setting.
 #
 # If a spam-checker extension is enabled, this variable's value is set automatically by the playbook during runtime.

roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -143,7 +143,7 @@ default_room_version: {{ matrix_synapse_default_room_version|to_json }}
 
 # Set to true to automatically forget rooms for users when they leave them, either
 # normally or via a kick or ban. Defaults to false.
-forget_rooms_on_leave: true
+forget_rooms_on_leave: {{ matrix_synapse_forget_rooms_on_leave | to_json }}
 
 # The GC threshold parameters to pass to `gc.set_threshold`, if defined
 #