[CI] Prepare CI for Merge Queues (#2308)

PR prepares CI to the GitHub Merge Queues. All github actions that were
running in PR adjusted so they can run in the merge queues. Zombienet
jobs will do nothing during PRs but they will run during merge queues.

Jobs that will be skipped during PR:
 - all zombienet jobs
 - all publish docker jobs

Jobs that will be skipped during merge queue:
 - check-labels
 - check-prdoc
 - pr-custom-review
 - review trigger

cc https://github.com/paritytech/ci_cd/issues/862

.github/workflows/pr-custom-review.yml

@@ -14,11 +14,15 @@ on:
       - ready_for_review
       - converted_to_draft
   pull_request_review:
+  merge_group:
 
 jobs:
   pr-custom-review:
     runs-on: ubuntu-latest
     steps:
+      - name: Skip merge queue
+        if: ${{ contains(github.ref, 'gh-readonly-queue') }}
+        run: exit 0
       - name: Skip if pull request is in Draft
         # `if: github.event.pull_request.draft == true` should be kept here, at
         # the step level, rather than at the job level. The latter is not