How to migrate my old android application to current version?

Question

I have developed on android application using gradle version 5.1.1

Now I'm trying to upgrade my android application to the current version.

I need to do changes in my application, Need to upgrade with the latest gradle version. How to upgrade my android application.

build.gradle:

buildscript {
    repositories {
        maven {
            url 'https://maven.fabric.io/public'
        }
        jcenter()
        maven {
            url 'https://maven.google.com/'
            name 'Google'
        }
        google()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.4.1'
        classpath "net.saliman:gradle-cobertura-plugin:2.4.0"
        classpath 'com.github.dcendents:android-maven-gradle-plugin:1.5'
        classpath 'com.jfrog.bintray.gradle:gradle-bintray-plugin:1.8.4'
        classpath 'com.novoda:bintray-release:0.9'
        classpath 'nl.javadude.gradle.plugins:license-gradle-plugin:0.11.0'
        classpath 'com.bmuschko:gradle-nexus-plugin:2.3.1'
        classpath 'com.google.gms:google-services:4.2.0'
        classpath 'io.fabric.tools:gradle:1.27.0'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {

    ext {
        libraryVersion = '1.0'
        mavenGroupId = 'com.abcd'
        siteUrl = 'https://github.com/txusballesteros/fit-chart'
        gitUrl = 'https://github.com/txusballesteros/fit-chart.git'

        compileSdkVersion = 28
        buildToolsVersion = "28.0.3"
        minSdkVersion = 21
        targetSdkVersion = 28
        supportLibraryVersion = "28.0.0"
        junitVersion = "4.12"
    }

    repositories {
        jcenter()
        maven {
            url 'https://maven.google.com/'
            name 'Google'
        }
    }
    repositories {
        maven {
            url "https://jitpack.io"
        }
        jcenter()
    }
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

subprojects {
    project.configurations.all {
        resolutionStrategy.eachDependency { details ->
            if (details.requested.group == 'com.android.support'
                    && !details.requested.name.contains('multidex') ) {
                details.useVersion "28.0.0"
            }
        }
    }
}

error image:

Anyone having idea about this, Please help me with the solution.