Skip e2e:test-on-cng job in stable branches

Update .qa:rules:e2e:test-on-cng rule to skip running in stable branches until https://gitlab.com/groups/gitlab-org/quality/quality-engineering/-/epics/89 is completed

.gitlab/ci/rules.gitlab-ci.yml

@@ -1758,6 +1758,8 @@
 
 .qa:rules:e2e:test-on-cng:
   rules:
+    - <<: *if-merge-request-targeting-stable-branch
+      when: never
     - <<: *if-security-merge-request
       when: never
     - <<: *if-security-schedule