Use deploy-pages@v4 (#2032)

Since we are using `upload-pages-artifact@v3` we must use v4 of the
deploy task (see
https://github.com/actions/upload-pages-artifact/releases/tag/v3.0.0)

Author: swernli

.github/workflows/publish-playground.yml

@@ -67,4 +67,4 @@ jobs:
     steps:
       - name: Deploy to GitHub Pages
         id: deployment
-        uses: actions/deploy-pages@v2
+        uses: actions/deploy-pages@v4