Update gradle-plugin sample to use Kotlin DSL

.github/workflow-samples/gradle-plugin/plugin/build.gradle.kts

@@ -0,0 +1,40 @@
+plugins {
+    `java-gradle-plugin`
+}
+
+repositories {
+    mavenCentral()
+}
+
+testing {
+    suites {
+        val test by getting(JvmTestSuite::class) {
+            useJUnitJupiter()
+        }
+
+        val functionalTest by registering(JvmTestSuite::class) {
+            dependencies {
+                implementation(project())
+            }
+
+            targets {
+                all {
+                    testTask.configure { shouldRunAfter(test) } 
+                }
+            }
+        }
+    }
+}
+
+gradlePlugin {
+    val greeting by plugins.creating {
+        id = "org.example.greeting"
+        implementationClass = "org.example.GradlePluginPlugin"
+    }
+}
+
+gradlePlugin.testSourceSets.add(sourceSets["functionalTest"])
+
+tasks.named<Task>("check") {
+    dependsOn(testing.suites.named("functionalTest"))
+}