[Gradle] Enable K2 support for sample compiler plugin

^KT-61106 Verification Pending

libraries/examples/kotlin-gradle-subplugin-example/src/main/kotlin/example/CompilerPlugin.kt

@@ -55,4 +55,6 @@ public class ExampleComponentRegistrar : ComponentRegistrar {
         val messageCollector = configuration.get(CLIConfigurationKeys.MESSAGE_COLLECTOR_KEY, MessageCollector.NONE)
         messageCollector.report(CompilerMessageSeverity.INFO, "Project component registration: $exampleValue")
     }
+
+    override val supportsK2: Boolean = true
 }