Bump actions/checkout from 5 to 6 (#192)

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

Author: dependabot[bot]

.github/workflows/ci-additional.yaml

@@ -20,7 +20,7 @@ jobs:
       run:
         shell: bash -l {0}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0 # Fetch all history for all branches and tags.
 

.github/workflows/ci.yaml

@@ -37,7 +37,7 @@ jobs:
 
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
 
     - name: Set environment variables
       run: |
@@ -87,7 +87,7 @@ jobs:
         python-version: ["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/upstream-dev-ci.yaml

@@ -37,7 +37,7 @@ jobs:
       matrix:
         python-version: ["3.13"]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0 # Fetch all history for all branches and tags.
       - name: Set up conda environment