Move all Matrix client apps into the matrix-addons container network

Putting each client into its own network was good for isolation,
but it's quite wasteful in terms of the container network pool.

roles/custom/matrix-client-element/tasks/validate_config.yml

@@ -6,7 +6,8 @@
       You need to define a required configuration setting (`{{ item }}`) for using Element.
   when: "vars[item] == ''"
   with_items:
-    - "matrix_client_element_default_hs_url"
+    - matrix_client_element_default_hs_url
+    - matrix_client_element_container_network
 
 - name: Fail if Element location sharing enabled, but no tile server defined
   ansible.builtin.fail: