Fix mutable classDiscriminatorMode in JsonConfiguration

Author: BenWoodworth

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

@@ -39,7 +39,7 @@ public class JsonConfiguration @OptIn(ExperimentalSerializationApi::class) inter
     @ExperimentalSerializationApi
     public val allowComments: Boolean = false,
     @ExperimentalSerializationApi
-    public var classDiscriminatorMode: ClassDiscriminatorMode = ClassDiscriminatorMode.POLYMORPHIC,
+    public val classDiscriminatorMode: ClassDiscriminatorMode = ClassDiscriminatorMode.POLYMORPHIC,
 ) {
 
     /** @suppress Dokka **/