Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/build.yml

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

.github/workflows/gradle-wrapper-validation.yml

@@ -6,5 +6,5 @@ jobs:
     name: "Validation"
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: gradle/wrapper-validation-action@v3