Upgrade to Gradle 6.8 and modernize the build

Closes gh-719

docs/src/test/java/com/example/Payload.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2016 the original author or authors.
+ * Copyright 2014-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,8 +27,7 @@ public class Payload {
 	@SuppressWarnings("unused")
 	public void bookFieldDescriptors() {
 		// tag::book-descriptors[]
-		FieldDescriptor[] book = new FieldDescriptor[] {
-				fieldWithPath("title").description("Title of the book"),
+		FieldDescriptor[] book = new FieldDescriptor[] { fieldWithPath("title").description("Title of the book"),
 				fieldWithPath("author").description("Author of the book") };
 		// end::book-descriptors[]
 	}