track v4, not v4.x.x, versions of github-pages-deploy action (#54)

The
[github-pages-deploy-action](https://github.com/JamesIves/github-pages-deploy-action)
appears to point `vX` tags to whatever the latest `vX.y.z` tag is (e.g.
[v4 currently points to the same commit that v4.6.8
does](https://github.com/JamesIves/github-pages-deploy-action/tags)),
which means we should be able to use `uses:
JamesIves/github-pages-deploy-action@v4` and always get the latest
v4.x.y that's available without having to merge dependabot update PRs
every few weeks.

.github/workflows/pkgdown.yaml

@@ -48,7 +48,7 @@ jobs:
 
       - name: Deploy to GitHub pages 🚀
         if: github.event_name != 'pull_request'
-        uses: JamesIves/github-pages-deploy-action@v4.6.8
+        uses: JamesIves/github-pages-deploy-action@v4
         with:
           clean: ${{ github.event_name == 'workflow_dispatch' && inputs.clean || false }}
           branch: gh-pages