[Java] Upgrade to Checkstyle 8.21 and Mockito 2.28.2.
[Java] Upgrade to Checkstyle 8.21 and Mockito 2.28.2.

PKRoma/simple-binary-encoding,real-logic/simple-binary-encoding,PKRoma/simple-binary-encoding,real-logic/simple-binary-encoding,real-logic/simple-binary-encoding,PKRoma/simple-binary-encoding,PKRoma/simple-binary-encoding,PKRoma/simple-binary-encoding,PKRoma/simple-binary-encoding,real-logic/simple-binary-encoding,real-logic/simple-binary-encoding,real-logic/simple-binary-encoding,PKRoma/simple-binary-encoding,real-logic/simple-binary-encoding