Build/Test Tools: Add branch filtering for Slack notifications workflow.

Since branch filtering happens prior to the workflow run being created, filtering the branches that `workflow_run` will fire on for this workflow should cut down on the number of skipped “Slack Notifications” runs listed in the Actions section of the repository.

This also removes the check for a `skipped` outcome in the requesting workflow. Workflows for push events resulting from WordPress Core commits are never skipped.

See #52644.

git-svn-id: https://develop.svn.wordpress.org/trunk@51558 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/slack-notifications.yml

@@ -17,6 +17,10 @@ on:
       - Test old branches
     types:
       - completed
+    branches:
+      - master
+      - trunk
+      - '[3-9].[0-9]'
 
 jobs:
   # Gathers the details needed for Slack notifications.
@@ -31,7 +35,7 @@ jobs:
   prepare:
     name: Prepare notifications
     runs-on: ubuntu-latest
-    if: ${{ github.repository == 'WordPress/wordpress-develop' && github.event.workflow_run.conclusion != 'skipped' && github.event.workflow_run.event != 'pull_request' }}
+    if: ${{ github.repository == 'WordPress/wordpress-develop' && github.event.workflow_run.event != 'pull_request' }}
     outputs:
       previous_conclusion: ${{ steps.previous-conclusion.outputs.previous_conclusion }}
       payload: ${{ steps.create-payload.outputs.payload }}