[BUG] discriminator property of supertype not marked as so in subtype

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?

Description

When I specify a type Pet containing a discriminator type and specify a subtype Dog using allOf, then the generated model value for Pet.type.isDiscriminator == true while Dog.type.isDiscriminator == false . I would expect Dog.type.isDiscriminator == true.

This behavior is unexpected to me, but I'm not 100% sure if it's intended or not. Can someone confirm? In case this is a bug I can invest some time to try to solve it.

openapi-generator version

5.3.0

OpenAPI declaration file content or url

    Pet:
      type: object
      discriminator:
        propertyName: type
      required: [name, type]
      properties:
        name:
          type: string
        type:
          type: string
    Dog:
      allOf:
        - $ref: "#/components/schemas/Pet"
        - type: object
          required: [name, type, age]
          properties:
            age:
              type: string

Generation Details

Using the folowing data_class.mustache template:

{{#allVars}}//{{&baseName}} isDiscriminator == {{isDiscriminator}}}
{{/allVars}}

You will be able to see in the generated models that for Pet, type.isDiscriminator == true, while for Dog the same is false.

Steps to reproduce

$ openapi-generator-cli generate -g kotlin -i openapi.yaml -o gen -t templates

Start with the OpenAPI declaration and the data_class.mustache template, then run the documented Kotlin generation command to reproduce the Pet and Dog output. Trace how the generated model metadata represents the inherited discriminator. Done means Dog.type is marked isDiscriminator == true, with the behavior covered by an appropriate regression test.