Enable testing merge queues @ GitHub Actions CI/CD

This allows org-hosted projects to start enabling merge queues in the
repository settings. With that, GitHub would trigger a separate event
against a merge commit derived from merging several pull requests with
the target branch.

.github/workflows/ci-cd.yml

@@ -2,6 +2,7 @@
 name: CI
 
 on:  # yamllint disable-line rule:truthy
+  merge_group:
   push:
     branches: ["main"]
   pull_request: