Switch from jcenter() to mavenCentral()

servlet/xml/java/helloworld/build.gradle

@@ -8,7 +8,7 @@ plugins {
 apply from: "gradle/gretty.gradle"
 
 repositories {
-	jcenter()
+	mavenCentral()
 	maven { url "https://repo.spring.io/milestone" }
 	maven { url "https://repo.spring.io/snapshot" }
 }