Try `useExperimentalAnnotation` if `optIn` not defined

mirai-console/tools/gradle-plugin/src/main/kotlin/MiraiConsoleGradlePlugin.kt

@@ -36,7 +36,12 @@ public class MiraiConsoleGradlePlugin : Plugin<Project> {
     }
 
     private fun KotlinSourceSet.configureSourceSet(project: Project, target: KotlinTarget) {
-        languageSettings.optIn("kotlin.RequiresOptIn")
+        try {
+            languageSettings.optIn("kotlin.RequiresOptIn")
+        } catch (e: NoSuchMethodError) {
+            @Suppress("DEPRECATION")
+            languageSettings.useExperimentalAnnotation("kotlin.RequiresOptIn")
+        }
         dependencies { configureDependencies(project, this@configureSourceSet, target) }
     }