[BUG] Cannot set systemProperties for the Gradle plugin

Bug Report Checklist

Description

Hello, I am trying to set the systemProperties field of the Gradle plugin's openApiGenerate task and am getting the following error:

Caused by: groovy.lang.MissingPropertyException: Could not set unknown property 'systemProperties' for extension 'openApiGenerate' of type org.openapitools.generator.gradle.plugin.extensions.OpenApiGeneratorGenerateExtension.
	at org.gradle.internal.metaobject.AbstractDynamicObject.setMissingProperty(AbstractDynamicObject.java:118)

I need this to set generateSupportingFiles to false. It seems that the maven plugin has this as a property but for Gradle it has to be done through a system property (is it better perhaps to ask for that as a feature instead?).

This is my Gradle task definition:

openApiGenerate {
    generatorName = "spring"
    inputSpec = "$rootDir/api/api-v1.0.yml".toString()
    outputDir = "$projectDir".toString()
    apiPackage = "com.example"
    modelPackage = "com.example"
    configOptions = [
            interfaceOnly: "true",
            openApiNullable: "false",
    ]
    systemProperties = [
            generateSupportingFiles: "false"
    ]
}

Thank you!

openapi-generator version

5.2.1
(could be a regression of https://github.com/OpenAPITools/openapi-generator/issues/551)

Steps to reproduce

Add my task configuration to a blank Gradle 7.1.1 project.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/551

Reproduce the configuration in a blank Gradle 7.1.1 project using the shown openApiGenerate task and confirm the MissingPropertyException. Read the Gradle plugin extension and compare its supported properties with the related issue #551; done means the requested systemProperties behavior or an explicitly documented alternative is covered by a regression test.