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