[BUG] [aspnetcore] useNewtonsoft option set to false leads to non running code generated

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 using the generator to create an asp.net webserver using aspnetcore generator, the option useNewtonsoft=false leads to a broken application (It does not compile, because Newronsoft.Json is still used in some places)

Following errors are shown:
test\src\Org.OpenAPITools\Startup.cs(62,26,62,44): error CS1061: 'JsonOptions' does not contain a definition for 'SerializerSettings' and no accessible extension method 'SerializerSettings' accepting a first argument of type 'JsonOptions' could be found (are you missing a using directive or an assembly reference?)
test\src\Org.OpenAPITools\Startup.cs(62,68,62,106): error CS0246: The type or namespace name 'CamelCasePropertyNamesContractResolver' could not be found (are you missing a using directive or an assembly reference?)
\test\src\Org.OpenAPITools\Startup.cs(63,26,63,44): error CS1061: 'JsonOptions' does not contain a definition for 'SerializerSettings' and no accessible extension method 'SerializerSettings' accepting a first argument of type 'JsonOptions' could be found (are you missing a using directive or an assembly reference?)
test\src\Org.OpenAPITools\Startup.cs(63,64,63,83): error CS0246: The type or namespace name 'StringEnumConverter' could not be found (are you missing a using directive or an assembly reference?)

openapi-generator version

6.3.0

OpenAPI declaration file content or url

openapi: "3.1.0"

info:
  title: enum-example
  version: "0.1"

servers:
  - url: /api

paths:
  /test:
    get:
      operationId: getEnum
      summary: Get enumeration value
      tags:
        - test
      responses:
        "200":
          description: OK
          content:
            application/json:
              schema:
                allOf:
                  - type: object
                    properties:
                      data:
                        $ref: "#/components/schemas/ExampleEnum"

components:
  schemas:
    ExampleEnum:
      type: string
      example: option1
      enum:
        - option1
        - option2
        - option3

Generation Details

Run openapi-generator-cli.cmd generate -i [PATH_TO_YAML] -g aspnetcore -o [OUTPUT_PATH] --additional-properties=useNewtonsoft=false,aspnetCoreVersion=6.0,compatibilityVersion=6.0,generateBody=false,classModifier=abstract,operationResultTask=true

Steps to reproduce

Run openapi-generator-cli.cmd generate -i [PATH_TO_YAML] -g aspnetcore -o [OUTPUT_PATH] --additional-properties=useNewtonsoft=false,aspnetCoreVersion=6.0

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/12952

Suggest a fix

Use System.Text.Json when useNewtonsoft is set to false or alternatively use System.Text.Json as default starting with .net core version 6.0

Start by reproducing the issue with the provided OpenAPI YAML and aspnetcore generation command using useNewtonsoft=false. Inspect the generated Startup.cs around the reported lines and trace the aspnetcore generator templates that produce them. Done means the generated .NET 6 application compiles without Newtonsoft.Json references when the option is disabled.