Fix test project is using deprecated JavaExec.main property

^KT-46019 In Progress

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/new-model-in-old-plugin/build.gradle.kts

@@ -42,7 +42,7 @@ kotlin.target.compilations {
 
 val runBenchmark by tasks.registering(JavaExec::class) {
     classpath = kotlin.target.compilations["benchmark"].run { runtimeDependencyFiles + output.allOutputs }
-    main = "com.example.ABenchmarkKt"
+    mainClass.set("com.example.ABenchmarkKt")
 }
 
 publishing {