Ruby 2 sync builds: Notify #backend instead of #f_ruby3

.gitlab-ci.yml

@@ -49,7 +49,7 @@ notify:
       when: on_failure
       allow_failure: true
   variables:
-    NOTIFY_CHANNEL: f_ruby3
+    NOTIFY_CHANNEL: backend
   script:
     - echo "NOTIFY_CHANNEL is ${NOTIFY_CHANNEL}"
     - echo "CI_PIPELINE_URL is ${CI_PIPELINE_URL}"