Bump actions/checkout from 5 to 6 in /.github/workflows

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] <[email protected]>

Author: dependabot[bot]

.github/workflows/build.yml

@@ -18,7 +18,7 @@ jobs:
         os: ['ubuntu-latest', 'macos-latest', 'windows-latest']
         python-version: ['3.10', '3.11', '3.12']
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v6
       with:

.github/workflows/cffconvert.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out a copy of the repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Check whether the citation metadata from CITATION.cff is valid
         uses: citation-file-format/[email protected]

.github/workflows/link-check.yml

@@ -13,7 +13,7 @@ jobs:
   linkChecker:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Link Checker
         id: lychee

.github/workflows/tests.yml

@@ -18,7 +18,7 @@ jobs:
         os: ['ubuntu-latest', 'macos-latest', 'windows-latest']
         python-version: ['3.10', '3.11', '3.12']
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v6
       with: