[Kotlin-spring] fix model inheritance

Created by: frecco75

This PR fixes kotlin classes generation with inherited models in the source schema.

For example given this schema

openapi: "3.0.1"
info:
  version: 1.0.0
  title: Users

paths:
  /users:
    get:
      responses:
        200:
          description: users
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/User'
  
  
components:
  schemas:
    User:
      type: object
      required:
        - name
      properties:
        name:
          type: string
      discriminator:
        propertyName: type

    Admin:
      title: Admin
      allOf:
        - $ref: '#/components/schemas/User'
        - type: object
          required:
            - role
          properties:
            role:
              type: string

With the fix the generated code is valid and compiles correctly.

Moreover, it's better to use JsonTypeInfo.As.EXISTING_PROPERTY to avoid duplicate name in the JSON response