Bump actions/checkout from 3 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

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

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/mkdocs.yml

@@ -35,7 +35,7 @@ jobs:
       - name: Prepare system
         run: pip install --user mkdocs mkdocs-material pymdown-extensions pygments markdown-blockdiag mkdocs-bibtex markdown-aafigure==v201904.0004 mkdocs-build-plantuml-plugin 'Pillow<10' 'Markdown<3.4'
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Setup Pages
         id: pages
         uses: actions/configure-pages@v4