use checkout v4 for cd workflow

v3 uses node 16, which triggers a deprecation warning in Github actions

https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

Author: schoblaska

.github/workflows/cd.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     if: ${{ github.event.workflow_run.conclusion == 'success' }}
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Tag and Push Gem
         id: tag-and-push-gem
         uses: discourse/publish-rubygems-action@v3