Allow skipping of Merge Train syncs

This adds support for skipping Merge Train syncs by adding the
"merge-train skip" tag to a commit message. This is necessary so we can
reduce the number of redundant sync pipelines as part of our release
process.

See https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/1258 for
more information.

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

@@ -676,10 +676,14 @@
 ##################
 .releases:rules:canonical-dot-com-gitlab-stable-branch-only:
   rules:
+    - if: '$CI_COMMIT_MESSAGE =~ /\[merge-train skip\]/'
+      when: never
     - if: '$CI_SERVER_HOST == "gitlab.com" && $CI_PROJECT_PATH == "gitlab-org/gitlab" && $CI_COMMIT_REF_NAME =~ /^[\d-]+-stable-ee$/'
 
 .releases:rules:canonical-dot-com-security-gitlab-stable-branch-only:
   rules:
+    - if: '$CI_COMMIT_MESSAGE =~ /\[merge-train skip\]/'
+      when: never
     - if: '$CI_SERVER_HOST == "gitlab.com" && $CI_PROJECT_PATH == "gitlab-org/security/gitlab" && $CI_COMMIT_REF_NAME =~ /^[\d-]+-stable-ee$/'
 
 #################