[BUG][Kotlin] When using additional properties as an array it doesn't specify List's type correctly

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When trying to specify additional properties to handle dynamic keys which will always be an array of a certain type, the generator doesn't specify the List's type correctly.

For the declaration below it generates model as following:

data class Item(val id: String, val type: String)

class Responses (


) : kotlin.collections.HashMap<String, kotlin.collections.List>()

Which is incorrect as the expected one would be something like this:

data class Item(val id: String, val type: String)

class Responses (


) : kotlin.collections.HashMap<String, kotlin.collections.List<Item>()

openapi-generator version

Gradle plugin 5.4.0

OpenAPI declaration file content or url

Responses:
  type: object
  additionalProperties:
    type: array
    items:
      $ref: '#/components/schemas/Item'

Item:
  type: object
  properties:
    id:
      type: string
    type:
      type: string

Generation Details

Gradle generator task config

group = "dependencies"
generatorName = "kotlin"
generateAliasAsModel = false
configOptions = [
        dateLibrary                      : "java8",
        library                          : "multiplatform",
        sourceFolder                     : "source/kotlin",
        sortModelPropertiesByRequiredFlag: "false",
        sortParamsByRequiredFlag         : "false",
        enumPropertyNaming               : "UPPERCASE",
        serializationLibrary             : "jackson",
        interfaceOnly                    : "true",
        modelMutable                     : "false"
]

Steps to reproduce

Generate the models using the above described spec

Related issues/PRs

NA

Suggest a fix

NA

Start by reproducing the Kotlin generation with the provided OpenAPI YAML and Gradle generator configuration, then inspect the generated Responses model and the Kotlin generator entry points that handle additionalProperties. Done means the generated declaration uses kotlin.collections.List rather than a raw kotlin.collections.List, with generation still succeeding for the example.