Add permissions for branch deletion in workflow

Added permissions to allow branch deletion.

Author: taladrane

.github/workflows/delete_staging_and_head_branches.yaml

@@ -8,6 +8,9 @@ on:
       - "advisories/**"
   workflow_dispatch:
 
+permissions:
+  contents: write   # Required to delete branches
+
 jobs:
   delete-staging-and-head-branches:
     if: ${{ !github.event.pull_request.head.repo.fork }}