[actions] Update github actions label and automerge. (sonic-net#11736)

1. Add auto approve step when adding label to version upgrading PR.
2. Use mssonicbld TOKEN to merge version upgrading PR instead of 'github actions'

.github/workflows/label.yml

@@ -20,6 +20,13 @@ jobs:
   label:
     runs-on: ubuntu-latest
     steps:
+    - name: approve
+      run: |
+        set -e
+        echo ${{ secrets.GITHUB_TOKEN }} | gh auth login --with-token
+        url=$(echo $GITHUB_CONTEXT | jq -r '.event.pull_request._links.html.href')
+        echo PR: $url
+        gh pr review $url --approve
     - uses: actions/labeler@main
       with:
         repo-token: "${{ secrets.GITHUB_TOKEN }}"