Create vaadin 7.1.15 widgetsets wit gradle 6.5.1

I have a vaadin 7.1.15 project and I added the followings tasks in my build.gradle to compile and clean widgetsets

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

task compileWidgetset(type: JavaExec, dependsOn: cleanWidgetset) {
    main = ''
    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: Gradle - Plugins 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???