matrix-appservice-irc: add deprecation notice for removed homeserver_media_url

The matrix_appservice_irc_homeserver_media_url variable was removed and
replaced by the new mediaProxy configuration. Users who had this variable
set will now get a helpful message pointing them to the new approach.

roles/custom/matrix-bridge-appservice-irc/tasks/validate_config.yml

@@ -44,6 +44,7 @@
     - {'old': 'matrix_appservice_irc_container_expose_client_server_api_port', 'new': '<superseded by matrix_appservice_irc_container_http_host_bind_port>'}
     - {'old': 'matrix_appservice_irc_container_self_build', 'new': 'matrix_appservice_irc_container_image_self_build'}
     - {'old': 'matrix_appservice_irc_docker_image_name_prefix', 'new': 'matrix_appservice_irc_docker_image_registry_prefix'}
+    - {'old': 'matrix_appservice_irc_homeserver_media_url', 'new': '<removed; media proxying now uses matrix_appservice_irc_ircService_mediaProxy_publicUrl>'}
 
 - name: Fail if matrix_appservice_irc_ircService_mediaProxy_publicUrl_pathPrefix does not start with a slash
   ansible.builtin.fail: