[BUG] Gradle 7, Kotlin build script - mutable type with setter

Bug Report Checklist

• [Y] Have you provided a full/minimal spec to reproduce the issue?
• [Y] Have you validated the input using an OpenAPI validator (example)?
• [Y] Have you tested with the latest master to confirm the issue still exists?
• [Y] Have you searched for related issues/PRs?
• [Y] What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Gradle open api generator plugin using Kotlin build script is broken with the gradle 7 upgrade. Gradle 7 has new validation rules on mutable type with setters and this breaks existing kotlin build scripts

openapi-generator version

5.1.0

OpenAPI declaration file content or url

petstore.yaml

Generation Details

Steps to reproduce

Run openApiValidate task

Related issues/PRs

Suggest a fix

https://docs.gradle.org/7.0.2/userguide/validation_problems.html#mutable_type_with_setter

Start by running the openApiValidate task with the Kotlin build script and petstore.yaml under Gradle 7. Read Gradle's mutable type with setter validation guidance and identify the existing build-script declaration that triggers it. Done means the task runs without the Gradle validation error.