Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v4...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>

.github/workflows/publish.yml

@@ -14,7 +14,7 @@ jobs:
        name: deploy
 
     steps:
-      - uses: "actions/checkout@v4"
+      - uses: "actions/checkout@v6"
       - uses: "actions/setup-python@v6"
         with:
           python-version: 3.9

.github/workflows/test-suite.yml

@@ -17,7 +17,7 @@ jobs:
         python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
 
     steps:
-      - uses: "actions/checkout@v4"
+      - uses: "actions/checkout@v6"
       - uses: "actions/setup-python@v6"
         with:
           python-version: "${{ matrix.python-version }}"