Disable danger in preparation branches

Most of these validations don't apply to preparation branches and they
cause a lot of noise in the merge request. Therefore disabling danger
when the branches look like branches that could be for a preparation MR.

.gitlab-ci.yml

@@ -453,6 +453,7 @@ danger-review:
       - master
     variables:
       - $CI_COMMIT_REF_NAME =~ /^ce-to-ee-.*/
+      - $CI_COMMIT_REF_NAME =~ /.*-stable(-ee)?-prepare-.*/
   script:
     - git version
     - danger --fail-on-errors=true