ci: 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/ci.yml

@@ -16,7 +16,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up JDK 21
         uses: actions/setup-java@v4
@@ -42,7 +42,7 @@ jobs:
   javadoc:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
@@ -55,7 +55,7 @@ jobs:
     runs-on: windows-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up JDK 21
         uses: actions/setup-java@v4

.github/workflows/dependency-review.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Dependency Review
         uses: actions/dependency-review-action@v4

.github/workflows/pages.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     environment: github-pages
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup Pages
         uses: actions/configure-pages@v4

.github/workflows/release.yml

@@ -17,7 +17,7 @@ jobs:
       RELEASE_VERSION: ${{ github.ref_name }}
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up JDK 21
         uses: actions/setup-java@v4