Migrate `-Xopt-in` to `-opt-in`

buildSrc/src/main/kotlin/ProjectConfigure.kt

@@ -64,7 +64,7 @@ fun Project.configureJvmTarget() {
     tasks.withType(KotlinCompile::class)
         .filter { it.name.contains("test", ignoreCase = true) }
         .forEach { task ->
-            task.kotlinOptions.freeCompilerArgs += "-Xopt-in=net.mamoe.mirai.utils.TestOnly"
+            task.kotlinOptions.freeCompilerArgs += "-opt-in=net.mamoe.mirai.utils.TestOnly"
         }
 
     extensions.findByType(JavaPluginExtension::class.java)?.run {