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] <support@github.com>

Author: dependabot[bot]

.github/workflows/tests.yml

@@ -7,7 +7,7 @@ jobs:
     name: Lint and typecheck Python code
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: citizensadvice/python-uv-setup-action@v1
       - uses: extractions/setup-just@v3
       - run: just lint
@@ -18,7 +18,7 @@ jobs:
     env:
       SLACK_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: citizensadvice/python-uv-setup-action@v1
       - uses: extractions/setup-just@v3
       - run: just all-tests
@@ -27,7 +27,7 @@ jobs:
     name: Ensure documentation is up to date
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: citizensadvice/python-uv-setup-action@v1
       - uses: extractions/setup-just@v3
       - run: just generate-docs