Ensure enum classes toString() returns kotlin.String (#16247)

When creating an enum where a valid value is `String`, the override of toString() will be the wrong type.

Simply using `*kotlin*.String` will allow using `String` as a value in an enum.

samples/client/petstore/kotlin-uppercase-enum/src/main/kotlin/org/openapitools/client/models/PetEnum.kt

@@ -48,7 +48,7 @@ enum class PetEnum(val value: kotlin.String) {
      * This solves a problem when the variable name and its value are different, and ensures that
      * the client sends the correct enum values to the server always.
      */
-    override fun toString(): String = value
+    override fun toString(): kotlin.String = value
 
     companion object {
         /**