[BUG] [kotlin-spring] interfaceOnly doesn't work as expected

Created by: mariusstokseth

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

Using the openapi-gradle plugin i setup a task to generate code for the Petstore example [YAML] (https://github.com/OAI/OpenAPI-Specification/blob/master/examples/v3.0/petstore.yaml). I have created the following task:

task buildKotlinClient(type: org.openapitools.generator.gradle.plugin.tasks.GenerateTask){
    generatorName = "kotlin-spring"
    inputSpec = "$rootDir/pet.yaml".toString()
    outputDir = "$rootDir/generated-sources/".toString()
    apiPackage = "sandbox.api"
    modelPackage = "sandbox.model"
    configOptions = [
            dateLibrary: "java8",
            swaggerAnnotations: "true",
            interfaceOnly: "true",
    ]
    systemProperties = [
            modelDocs: "false",
            generateSupportingFiles: "false",
    ]
}

From what I understand only the PetAPI and the objects Error and Pet should be generated. Instead the task also generates Application.kt, build.gradle.kts, pom.xml, README.m, settings.gradle and application.yaml

openapi-generator version 4.2.2

gradle version 6.1.1

OpenAPI declaration file content or url

URL: https://github.com/OAI/OpenAPI-Specification/blob/master/examples/v3.0/petstore.yaml

Steps to reproduce

Download Petstore Example YAML
Run code generation task using the Gradle Plugin
Navigate to the genearted-sources folder