Bump peaceiris/actions-gh-pages from 3.9.3 to 4.0.0 (#188)

Bumps [peaceiris/actions-gh-pages](https://github.com/peaceiris/actions-gh-pages) from 3.9.3 to 4.0.0.
- [Release notes](https://github.com/peaceiris/actions-gh-pages/releases)
- [Changelog](https://github.com/peaceiris/actions-gh-pages/blob/main/CHANGELOG.md)
- [Commits](peaceiris/actions-gh-pages@373f7f2...4f9cc66)

---
updated-dependencies:
- dependency-name: peaceiris/actions-gh-pages
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/deploy.yml

@@ -30,7 +30,7 @@ jobs:
         run: make all
 
       - name: Deploy to seismo-learn.github.io
-        uses: peaceiris/actions-gh-pages@373f7f263a76c20808c831209c920827a82a2847
+        uses: peaceiris/actions-gh-pages@4f9cc6602d3f66b9c108549d475ec49e8ef4d45e
         with:
           # personal token is needed here because it's pushing to an external repository.
           # personal token can be generated at https://github.com/settings/tokens,