Add additional-networks support to matrix-coturn

Not that it seems necessary right now, but it makes it consistent with
all other roles.

roles/custom/matrix-coturn/tasks/validate_config.yml

@@ -1,5 +1,14 @@
 ---
 
+- name: (Deprecation) Catch and report renamed Coturn settings
+  ansible.builtin.fail:
+    msg: >-
+      Your configuration contains a variable, which now has a different name.
+      Please change your configuration to rename the variable (`{{ item.old }}` -> `{{ item.new }}`).
+  when: "item.old in vars"
+  with_items:
+    - {'old': 'matrix_coturn_docker_network', 'new': 'matrix_coturn_container_network'}
+
 - name: Fail if required Coturn settings not defined
   ansible.builtin.fail:
     msg: >-