[Java] Upgrade to Checkstyle 8.21 and Mockito 2.28.2.

mjpt777 · mjpt777 · commit 1588387980c0 · 2019-06-01T11:43:27.000+01:00
diff --git a/build.gradle b/build.gradle
@@ -145,7 +145,7 @@ subprojects {
     }
 
     checkstyle {
-        toolVersion = '8.20'
+        toolVersion = '8.21'
     }
 
     compileJava {
