ci: ⬆️ bump actions/checkout from 5 to 6 (#130)

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/api-docs-release.yml

@@ -14,7 +14,7 @@ jobs:
     name: API Docs Release
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Set up Python
         uses: actions/setup-python@v6

.github/workflows/pypi-release.yml

@@ -14,7 +14,7 @@ jobs:
     needs: [test]
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Set up Python 3.9
         uses: actions/setup-python@v6

.github/workflows/python-build.yml

@@ -12,7 +12,7 @@ jobs:
         python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v6

.github/workflows/python-lint.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Set up Python
         uses: actions/setup-python@v6

.github/workflows/python-test.yml

@@ -12,7 +12,7 @@ jobs:
         python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v6