[BUG] [csharp-netcore] ToJson is not marked virtual even though it's overridden

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

For an OpenAPI declaration with two component schemas, one of which includes properties form the other using allOf, when using the csharp-netcore generator, OpenAPI Generator will generate a non-virtual ToJson method in a base class that is overridden in a derived class. This seems to happen when the base schema has additionalProperties defined.

For the OpenAPI declaration file below, I would expect OpenAPI Generator to generate the following method declaration in BaseClass:

public virtual string ToJson()
{
   return JsonConvert.SerializeObject(this, Formatting.Indented);
}

However, it actually generates this one:

public string ToJson()
{
   return JsonConvert.SerializeObject(this, Formatting.Indented);
}

In the DerivedClass definition (public partial class DerivedClass : BaseClass), this leads to the ToJson method definition being invalid:

public override string ToJson()
{
    return JsonConvert.SerializeObject(this, Formatting.Indented);
}

The compiler error generated is: Error CS0506 'DerivedClass.ToJson()': cannot override inherited member 'BaseClass.ToJson()' because it is not marked virtual, abstract, or override Org.OpenAPITools {PATH}\src\Org.OpenAPITools\Model\DerivedClass.cs line 64

openapi-generator version

Version 4.2.3

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: ToJson Virtual Bug
  version: '1.0'
servers:
  - url: localhost
paths:
  /test-path:
    get:
      responses:
        '200':
          description: Success
components:
  schemas:
    BaseClass:
      required:
        - $type
      type: object
      properties:
        $type:
          type: string
      additionalProperties:
        type: object
        additionalProperties: false
      discriminator:
        propertyName: $type
    DerivedClass:
      type: object
      allOf:
        - $ref: '#/components/schemas/BaseClass'

Command line used for generation

npx -p @openapitools/openapi-generator-cli@cli-4.2.3 openapi-generator generate -i ./tojsonvirtualbug.yaml -g csharp-netcore -o ./tojsonvirtualbug

Steps to reproduce

Related issues/PRs

Suggest a fix

OpenAPI Generator could always output the virtual keyword if the override keyword is not generated. I believe that change would happen here.

Start with modules/openapi-generator/src/main/resources/csharp-netcore/modelGeneric.mustache at the referenced line, then generate the supplied OpenAPI declaration with the csharp-netcore command. Confirm that BaseClass.ToJson is virtual when DerivedClass overrides it, and verify the generated C# output no longer produces compiler error CS0506.