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] <[email protected]>

Author: dependabot[bot]

.github/workflows/run_tests_matlab.yml

@@ -37,7 +37,7 @@ jobs:
         release: ${{ matrix.version }}
 
     - name: Clone spm_2_bids
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         submodules: true
         fetch-depth: 2

.github/workflows/run_tests_octave.yml

@@ -25,7 +25,7 @@ jobs:
         sudo apt-get -y install octave octave-dev
 
     - name: Clone spm_2_bids
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         submodules: true
         fetch-depth: 2

.github/workflows/validate_cff.yml

@@ -19,7 +19,7 @@ jobs:
 
     steps:
 
-    - 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]