JSON: Fix mutable classDiscriminatorMode in config, and mark experimental in builder (#2680)

Since this is a new feature, it should have been added as experimental to the builder, but it wasn't done properly. 

Deprecate mutable `classDiscriminatorMode` in `JsonConfiguration` — vars are not needed there.

Author: BenWoodworth

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

@@ -502,6 +502,7 @@ public class JsonBuilder internal constructor(json: Json) {
      * Other modes are generally intended to produce JSON for consumption by third-party libraries,
      * therefore, this setting does not affect the deserialization process.
      */
+    @ExperimentalSerializationApi
     public var classDiscriminatorMode: ClassDiscriminatorMode = json.configuration.classDiscriminatorMode
 
     /**

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

@@ -39,6 +39,11 @@ public class JsonConfiguration @OptIn(ExperimentalSerializationApi::class) inter
     @ExperimentalSerializationApi
     public val allowComments: Boolean = false,
     @ExperimentalSerializationApi
+    @set:Deprecated(
+        "JsonConfiguration is not meant to be mutable, and will be made read-only in a future release. " +
+            "The `Json(from = ...) {}` copy builder should be used instead.",
+        level = DeprecationLevel.ERROR
+    )
     public var classDiscriminatorMode: ClassDiscriminatorMode = ClassDiscriminatorMode.POLYMORPHIC,
 ) {