Bump peaceiris/actions-gh-pages from 3 to 4

Bumps [peaceiris/actions-gh-pages](https://github.com/peaceiris/actions-gh-pages) from 3 to 4.
- [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@v3...v4)

---
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]>

.github/workflows/gh-pages.yml

@@ -32,7 +32,7 @@ jobs:
             - run: mdbook build docs
 
             - name: Deploy
-              uses: peaceiris/actions-gh-pages@v3
+              uses: peaceiris/actions-gh-pages@v4
               with:
                   github_token: ${{ secrets.GITHUB_TOKEN }}
                   publish_dir: ./docs/book/