chore: Use Gradle 6 built-in support for javadoc and source jars (appium#1301)

https://docs.gradle.org/6.0/release-notes.html#javadoc-sources-jar

Author: valfirst

build.gradle

@@ -37,10 +37,14 @@ dependencies {
  lombok 'org.projectlombok:lombok:1.18.8'
 }
 
-compileJava {
- sourceCompatibility = 1.8
- targetCompatibility = 1.8
+java {
+ sourceCompatibility = JavaVersion.VERSION_1_8
+ targetCompatibility = JavaVersion.VERSION_1_8
+ withJavadocJar()
+ withSourcesJar()
+}
 
+compileJava {
  def ecjJar = configurations.ecj.singleFile
  def lombokjar = configurations.lombok.singleFile
 
@@ -117,16 +121,6 @@ checkstyle {
 }
 checkstyleMain.excludes = ['**/org/openqa/selenium/**']
 
-task sourcesJar(type: Jar) {
- from sourceSets.main.allJava
- archiveClassifier = 'sources'
-}
-
-task javadocJar(type: Jar) {
- from javadoc
- archiveClassifier = 'javadoc'
-}
-
 javadoc {
  options.addStringOption('encoding', 'UTF-8')
 }