Upgrade to Gradle 6.8 and modernize the build

Closes gh-719

docs/src/test/java/com/example/restassured/CustomEncoding.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2017 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.
@@ -37,8 +37,7 @@ public class CustomEncoding {
 	public void setUp() {
 		// tag::custom-encoding[]
 		this.spec = new RequestSpecBuilder()
-				.addFilter(documentationConfiguration(this.restDocumentation)
-						.snippets().withEncoding("ISO-8859-1"))
+				.addFilter(documentationConfiguration(this.restDocumentation).snippets().withEncoding("ISO-8859-1"))
 				.build();
 		// end::custom-encoding[]
 	}