chore(deps): 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/build-test-publish.yml

@@ -43,7 +43,7 @@ jobs:
       id-token: write
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: hynek/build-and-inspect-python-package@v2
@@ -85,7 +85,7 @@ jobs:
             dependencies: "pre"
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: actions/cache@v4
       with:
         path: /var/lib/apt

.github/workflows/docs-build-update.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         ssh-key: "${{ secrets.NIPREPS_DEPLOY }}"
         fetch-depth: 0

.github/workflows/validate.yml

@@ -20,12 +20,12 @@ jobs:
   style:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - run: pipx run ruff check sdcflows/
     - run: pipx run ruff format --diff sdcflows/
 
   # codespell:
   #   runs-on: ubuntu-latest
   #   steps:
-  #   - uses: actions/checkout@v5
+  #   - uses: actions/checkout@v6
   #   - uses: codespell-project/actions-codespell@v2