Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Checkout
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
 
     - name: Gradle Wrapper Validation
       uses: gradle/actions/wrapper-validation@v5
@@ -45,7 +45,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Configure JDK
         uses: actions/setup-java@v5