Bump actions/checkout from 4 to 6 (#54)

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/build-and-publish.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Set up Git repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: release.sh
         # We need java, pandoc, and more, which is not provided directly by the image
         uses: dante-ev/latex-action@edge
@@ -43,7 +43,7 @@ jobs:
           wget https://github.com/plantuml/plantuml/releases/download/v1.2025.1/plantuml-1.2025.1.jar
       - name: Set PLANTUML_JAR
         run: echo "PLANTUML_JAR=/tmp/plantuml/plantuml-1.2025.1.jar" >> "$GITHUB_ENV"
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: awalsh128/cache-apt-pkgs-action@latest
         with:
           packages: graphviz inkscape