Update docs CI gh-pages version

Node.js 16 is deprecated. Updating this to the latest version so that it continues working.

Author: dmadison

.github/workflows/docs.yml

@@ -46,7 +46,7 @@ jobs:
           doxygen Doxyfile
 
       - name: Deploy Docs
-        uses: peaceiris/actions-gh-pages@v3
+        uses: peaceiris/actions-gh-pages@v4
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_branch: gh-pages