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/ci-format.yml

@@ -15,7 +15,7 @@ jobs:
     name: Format
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: actions/setup-python@v6
       with:
         python-version: '3.10'

.github/workflows/reusable_ici.yml

@@ -49,10 +49,10 @@ jobs:
     steps:
       - name: Checkout ${{ inputs.ref }} when build is not scheduled
         if: ${{ github.event_name != 'schedule' }}
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Checkout ${{ inputs.ref }} on scheduled build
         if: ${{ github.event_name == 'schedule' }}
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           ref: ${{ inputs.ref_for_scheduled_build }}
       - name: cache target_ws

.github/workflows/sphinx_build.yml

@@ -17,7 +17,7 @@ jobs:
     concurrency:
       group: ${{ github.workflow }}-${{ github.ref }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: actions/setup-python@v6
         with:
           cache: 'pip'