Add opt-in to JsonConfiguration() constructor call

becuase KT-55074 is fixed in 1.8.20+

Author: sandwwraith

formats/json/commonMain/src/kotlinx/serialization/json/Json.kt

@@ -67,6 +67,7 @@ public sealed class Json(
      * The default instance of [Json] with default configuration.
      */
     @ThreadLocal // to support caching
+    @OptIn(ExperimentalSerializationApi::class)
     public companion object Default : Json(JsonConfiguration(), EmptySerializersModule())
 
     /**