Build/Test Tools: Add branch filtering to upgrade test workflow.

The upgrade testing workflow is only meant to run from `trunk`. When a pull request as any other BASE branch, the workflow should not run.

See #63170.

git-svn-id: https://develop.svn.wordpress.org/trunk@60767 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

.github/workflows/upgrade-testing.yml

@@ -12,6 +12,9 @@ on:
       - '.github/workflows/upgrade-testing.yml'
       - '.github/workflows/reusable-upgrade-testing.yml'
   pull_request:
+    # This workflow is only meant to run from trunk. Pull requests changing this file with different BASE branches should be ignored.
+    branches:
+      - trunk
     # Always test the workflow when changes are suggested.
     paths:
       - '.github/workflows/upgrade-testing.yml'