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](actions/checkout@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>

Author: dependabot[bot]

.github/workflows/code-quality.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
@@ -32,7 +32,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:

.github/workflows/javadoc.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Set up JDK 25
         uses: actions/setup-java@v4
         with:

.github/workflows/publish.yml

@@ -13,7 +13,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up JDK 21
         uses: actions/setup-java@v4

.github/workflows/unit-tests.yml

@@ -14,7 +14,7 @@ jobs:
         java-version: [ '25' ]
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Set up JDK ${{ matrix.java-version }}
         uses: actions/setup-java@v4
         with:
@@ -39,7 +39,7 @@ jobs:
         java-version: [ '25' ]
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Set up JDK ${{ matrix.java-version }}
         uses: actions/setup-java@v4
         with: