Skip Danger checks on stable branches

When MR's are picked into stable branches, Danger reviews are also
performed, this causes for the prep MRs to fail as they don't satisfy
the Danger reviews, since these ones are not necessary, we should skip
them on stable branches

gitlab-ci-config/gitlab-com.yml

@@ -167,6 +167,7 @@ danger-review:
   except:
     refs:
       - schedules@gitlab-org/omnibus-gitlab
+      - /^[\d-]+-stable(-ee)?$/
     variables:
       - $DEPS_PIPELINE
   script: