Issue #142: resolve jdk version error

lazybox/build.gradle.kts

@@ -5,6 +5,8 @@
  * For more details on building Java & JVM projects, please refer to https://docs.gradle.org/8.3/userguide/building_java_projects.html in the Gradle documentation.
  */
 
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
 plugins {
     kotlin("jvm") version "1.9.22"
     application
@@ -29,10 +31,16 @@ dependencies {
     testRuntimeOnly("org.junit.platform:junit-platform-launcher")
 }
 
-// Apply a specific Java toolchain to ease working on different environments.
 java {
-    toolchain {
-        languageVersion.set(JavaLanguageVersion.of(11))
+    sourceCompatibility = JavaVersion.VERSION_11
+    targetCompatibility = JavaVersion.VERSION_11
+}
+
+tasks.withType<KotlinCompile>().all {
+    kotlinOptions {
+        freeCompilerArgs += "-opt-in=kotlin.RequiresOptIn"
+        freeCompilerArgs += "-opt-in=kotlin.ExperimentalUnsignedTypes"
+        jvmTarget = "11"
     }
 }