workflows: update trigger for merge queue event

The trigger for the merge queue events for CI was not set up properly.
This is the proper way it should be done according to the documentation.

Epic: none
Part of: DEVINF-1127
Release note: None
Release justification: Non-production code changes

.github/workflows/github-actions-essential-ci.yml

@@ -1,5 +1,6 @@
 name: GitHub Actions Essential CI
 on:
+  merge_group:
   pull_request:
     types: [opened, reopened, synchronize]
     branches:
@@ -25,7 +26,6 @@ on:
       - "!staging-v23.2*"
   push:
     branches:
-      - "gh-readonly-queue/*"
       - "master"
       - "release-*"
       - "staging-*"