CI: 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/tests.yml

@@ -19,7 +19,7 @@ jobs:
     permissions: {}
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: actions/setup-node@v6
       with:
         node-version: ${{ matrix.node-version }}
@@ -32,7 +32,7 @@ jobs:
     permissions: {}
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: actions/setup-node@v6
       - name: Install dependencies
         run: npm ci
@@ -50,7 +50,7 @@ jobs:
       pull-requests: write # allow comments
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: actions/setup-node@v6
     - run: npm ci
     - run: npm run test-type-definitions
@@ -64,7 +64,7 @@ jobs:
       pull-requests: write # allow comments
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: actions/setup-node@v6
     - run: npm ci
     - run: npm run lint