Update java-toolchain sample to use Kotlin DSL

.github/workflow-samples/java-toolchain/build.gradle.kts

@@ -0,0 +1,21 @@
+plugins {
+    java
+}
+
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(16)
+    }
+}
+
+repositories {
+    mavenCentral()
+}
+
+testing {
+    suites {
+        val test by getting(JvmTestSuite::class) {
+            useJUnit()
+        }
+    }
+}