[Java] Upgrade to Checkstyle 8.39 and Mockito 3.7.0.

Author: mjpt777

build.gradle

@@ -45,9 +45,9 @@ def javaLauncher = javaToolchains.launcherFor {
     languageVersion = JavaLanguageVersion.of(buildJavaVersion)
 }
 
-def checkstyleVersion = '8.38'
+def checkstyleVersion = '8.39'
 def hamcrestVersion = '2.2'
-def mockitoVersion = '3.6.28'
+def mockitoVersion = '3.7.0'
 def junitVersion = '5.7.0'
 def jmhVersion = '1.27'
 def agronaVersion = '1.9.0'