github/workflows: revert to github-script@v6

There is an upstream regression in v7. Use v6 until Node 16 is still
available.

See: actions/github-script#448

.github/workflows/comment.yml

@@ -10,7 +10,7 @@ jobs:
     if: github.event.workflow_run.event == 'pull_request' && github.event.workflow_run.conclusion == 'success'
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/github-script@v7
+      - uses: actions/github-script@v6
         with:
           script: |
             const { owner, repo } = context.repo;