Bump actions/checkout from 5 to 6 (#88)

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

Author: dependabot[bot]

.github/workflows/matlab_test_and_coverage.yaml

@@ -38,7 +38,7 @@ jobs:
       with:
         release: ${{ matrix.version }}
 
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         submodules: true
         fetch-depth: 1

.github/workflows/octave_test_and_coverage.yml

@@ -25,7 +25,7 @@ jobs:
         sudo apt-get -y -qq update
         sudo apt-get -y install octave octave-dev
 
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         submodules: true
         fetch-depth: 1

.github/workflows/validation.yml

@@ -16,7 +16,7 @@ jobs:
   validate_cff:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Check whether the citation metadata from CITATION.cff is valid
       uses: citation-file-format/cffconvert-github-action@2.0.0
       with: