Create vaadin 7.1.15 widgetsets wit gradle 6.5.1

I am trying to generate the widgetsets with gradle as follows

task cleanWidgetset(type: Delete) {
	delete webAppDirName+'/VAADIN/widgetsets'
}

task compileWidgetset(type: JavaExec, dependsOn: cleanWidgetset) {
	main = 'com.vaadin.tools.WidgetsetCompiler'
	classpath sourceSets.main.resources.srcDirs
	classpath configurations.compile
	//classpath configurations.gwt
	args '-war'
	args webAppDirName+'/VAADIN/widgetsets'
	args '-logLevel'
	args 'INFO'
	args '-deploy'
	args '/tmp/widgetset_widgetset.IrmoWidgetset'
	args '-extra'
	args '/tmp/widgetset_widgetset.IrmoWidgetset'
	args '-localWorkers'
	args '8'
	args 'widgetset.IrmoWidgetset'
	jvmArgs '-Xmx1024M'
	jvmArgs '-Xss512M'
	jvmArgs '-Djava.awt.headless=true'
}

Unfortunately when I run ./gradlew compileWidgetset war I am receiving the following error: error: package com.vaadin.flow.component does not exist

I also tried apply any gradle plugins of the following list: https://plugins.gradle.org/search?term=vaadin but unfortunately the conbination between gradle version (6.5.1) and vaadin version (7.1.15) makes it not possible to apply any of them.

any idea how I can do to generate the widgetsets with that combination of versions of gradle and vaadin???