Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...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>

Author: dependabot[bot]

.github/workflows/auto-release.yml

@@ -11,7 +11,7 @@ jobs:
       run: |
         Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
         echo "DISPLAY=:99" >> $GITHUB_ENV
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: detect version
       uses: EndBug/version-check@v2
       id: check

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
       run: |
         Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
         echo "DISPLAY=:99" >> $GITHUB_ENV
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - run: npm ci
     - run: npm test
     - name: clang-format (diff)

.github/workflows/unstable-release.yml

@@ -13,7 +13,7 @@ jobs:
       run: |
         Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
         echo "DISPLAY=:99" >> $GITHUB_ENV
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: read version
       id: version
       uses: martinbeentjes/npm-get-version-action@v1.3.1