Revert "Change the trigger to check the CLA" (#20568)

Reverts #20567

It turns out to be a bad idea, PRs only keep the status of the jobs of
the last commit. Therefore, we will always have a the CLA job that is
not reported...

I've played around in a different repository to test which is best but
this new trigger is very much useless in our use case.

.github/workflows/cla.yml

@@ -1,15 +1,22 @@
-name: Check Scala CLA
+name: Scala CLA
 on:
   pull_request:
-    types: opened
     branches-ignore:
       - 'language-reference-stable'
+  push:
+    branches:
+      - 'language-reference-stable'
+  merge_group:
+permissions:
+  contents: write
+  pull-requests: write
 
 jobs:
   check:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
     - run: ./project/scripts/check-cla.sh
+      if: github.event_name == 'pull_request'
       env:
         AUTHOR: ${{ github.event.pull_request.user.login }}