Update actions/checkout action to v4 (#72)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/gradle.yml

@@ -20,7 +20,7 @@ jobs:
     name: Gradle wrapper validation
     steps:
       - name: Check out repo
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Validate Gradle wrapper
         uses: gradle/actions/wrapper-validation@v4