ZOOKEEPER-4048: Upgrade Mockito to 3.6.28 - allow builds on JDK16

Author: Enrico Olivelli <eolivelli@apache.org>

Reviewers: Damien Diederen <ddiederen@apache.org>

Closes #1573 from eolivelli/fix/ZOOKEEPER-4048

pom.xml

@@ -434,7 +434,7 @@
     <junit.version>5.6.2</junit.version>
     <junit-platform.version>1.6.2</junit-platform.version>
     <log4j.version>1.2.17</log4j.version>
-    <mockito.version>2.28.2</mockito.version>
+    <mockito.version>3.6.28</mockito.version>
     <hamcrest.version>2.2</hamcrest.version>
     <commons-cli.version>1.4</commons-cli.version>
     <netty.version>4.1.50.Final</netty.version>