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/automerge.yml

@@ -11,7 +11,7 @@ jobs:
     if: github.actor == 'dependabot[bot]'
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           show-progress: ''
       - name: Merge PR

.github/workflows/check.yml

@@ -19,7 +19,7 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Check test/consistent.bib
         uses: ./
         with:
@@ -34,7 +34,7 @@ jobs:
         if: steps.inconsistent.outcome == 'success'
         run: exit 1
 
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           repository: JabRef/jabref-demonstration-libraries
           path: jabref-demonstration-libraries