[actions] Add permissions block to the 'update-single-platform-branch…

…es' action. (#20441)

It only took me two and a half months to notice the action wasn't working...

.github/workflows/update-single-platform-branches.yml

@@ -11,6 +11,11 @@ jobs:
  updateSinglePlatformBranches:
  name: Merge main into single-platform release test branches
  runs-on: ubuntu-latest
+ # GITHUB_TOKEN change from read-write to read-only on 2024-02-01 requires permissions block
+ # https://docs.opensource.microsoft.com/github/apps/permission-changes/
+ # https://docs.github.com/en/actions/using-jobs/assigning-permissions-to-jobs
+ permissions:
+ contents: write
 
  steps:
  - name: Checkout repo