[BUG][kotlin-spring] Generating schema model with additional properties in component leads to compile error

Description

Generating schema model with additional properties leads to compile error.

Steps to reproduce:
swagger.yaml:

JsonObjectItem:
      description: example item of Json Object (could be anything)
      properties:
        something1:
          type: string
        something2:
          type: string
      additionalProperties:
        type: string

generates

`data class JsonObjectItem (

    @JsonProperty("something1") val something1: kotlin.String? = null,

    @JsonProperty("something2") val something2: kotlin.String? = null

) : kotlin.mapOf<String, kotlin.String>{

}`

which leads to

JsonObjectItem.kt: (23, 12): Unresolved reference: mapOf

openapi-generator version

4.1.3

OpenAPI declaration file content or url

--> https://gist.github.com/Alexichlol/a4ce6f2a3742cc28a757fa736923a436

Command line used for generation

gradle plugin

Steps to reproduce

in description but basically just generate code and try to compile

Related issues/PRs

Suggest a fix

don't really know the idea behind current implementation but maybe either not using data class if want to inherit from map and inherit from hashmap like in spring java or use composition and have map as the field

Start with the swagger.yaml schema and the generated JsonObjectItem.kt from the linked reproduction, then locate the Kotlin Spring generator entry point that handles additionalProperties. Reproduce generation with the Gradle plugin and compile the result. Done means the model generated for this schema compiles while retaining the intended additional-properties behavior.