Stop using deprecated matrix_bot_postmoogle_domain variable in group vars

group_vars/matrix_servers

@@ -2306,7 +2306,7 @@ matrix_ssl_domains_to_obtain_certificates_for: |
     +
     ([matrix_server_fqn_ntfy] if matrix_ntfy_enabled else [])
     +
-    ([matrix_bot_postmoogle_domain] if matrix_bot_postmoogle_enabled else [])
+    (matrix_bot_postmoogle_domains if matrix_bot_postmoogle_enabled else [])
     +
     ([matrix_domain] if matrix_nginx_proxy_base_domain_serving_enabled else [])
     +