Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/build-test.yml

@@ -6,7 +6,7 @@ jobs:
     name: Build with Java 8
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
       - uses: actions/setup-java@v3
         with:
           java-version: "8"