Fixed notifications for build failures

no issue

- we were still referencing `master` here, whereas we've since switched
  to `main`
- this commit updates the workflow to trigger the notification if it
  fails on `main`
- also enables testing on `v4.*` branches

ghost/admin/.github/workflows/test.yml

@@ -3,6 +3,7 @@ on:
   pull_request:
   push:
     branches:
+      - 'v4.*'
       - main
       - 'renovate/*'
 jobs:
@@ -20,7 +21,7 @@ jobs:
       - run: grunt shell:ember:prod
 
       - uses: daniellockyer/action-slack-build@master
-        if: failure() && github.event_name == 'push' && (github.ref == 'refs/heads/4.0' || github.ref == 'refs/heads/master')
+        if: failure() && github.event_name == 'push' && github.ref == 'refs/heads/main'
         with:
           status: ${{ job.status }}
         env: