[BUG][gradle plugin] User-defined templates do not work with the gradle plugin

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• 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

Following the instructions from the documentation:
https://github.com/OpenAPITools/openapi-generator/blob/master/docs/templating.md

I got the CLI, created the following config.json file:

{
  "generatorName": "typescript-angular",
  "inputSpec": "spec.json",
  "outputDir": "gen",
  "templateDir": "templates",
  "files": {
    "AUTHORS.md": {}
  }
}

Created the templates directory, added an empty AUTHORS.md file and executed the following command:
openapi-generator-cli generate -c config.json

This resulted in the expected generated typescript projet, witht he additional AUTHORS.md file.

Afterwards I tried to make the same change to my gradle configuration:

    task("openApiGenerateClient_spec", type: GenerateTask, group: 'openapi subtasks') {
        generatorName = "typescript-angular"
        inputSpec = "spec.json"
        outputDir = "gen"
        // lines below are newly added
        templateDir = "templates"
        files = {
            AUTHORS.md = {}
        }
    }

However, when I now run this task, I get the following error:

> Could not set unknown property 'files' for task ':openApiGenerateClient_spec' of type org.openapitools.generator.gradle.plugin.tasks.GenerateTask.

Which corresponds to the GenerateTask not having a files property:
https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator-gradle-plugin/src/main/kotlin/org/openapitools/generator/gradle/plugin/tasks/GenerateTask.kt

openapi-generator version

6.2.0

Start with modules/openapi-generator-gradle-plugin/src/main/kotlin/org/openapitools/generator/gradle/plugin/tasks/GenerateTask.kt and compare its configuration with the templating documentation and the working CLI config. Run the Gradle task from the reported example; done means the Gradle plugin accepts templateDir and files and generates AUTHORS.md.