ci: adjust triggers for gh merge queues

For merge queue's to work the CI has to run on the `merge_group` event.
Also ignore pushes to the merge queue branches to avoid running CI twice

Author: sjoerdsimons

.github/workflows/ci.yml

@@ -1,6 +1,10 @@
-on: [push, pull_request]
-
 name: CI
+on:
+  push:
+    branches-ignore:
+      - "gh-readonly-queue/**"
+    pull_request:
+    merge_group:
 
 env:
   REGISTRY: ghcr.io