Update dependency org.mockito:mockito-core to v3.1.0 (googleapis#2244)

pom.xml

@@ -71,7 +71,7 @@ limitations under the License.
 
         <!-- testing dependency versions -->
         <junit.version>4.12</junit.version>
-        <mockito.version>3.0.0</mockito.version>
+        <mockito.version>3.1.0</mockito.version>
         <!-- TODO: check if commons-codec was transitively updated to 1.13 and okhttp was updated to 2.7.5 when upgrading-->
         <beam.version>2.15.0</beam.version>
         <!-- referred from bigtable-beam-import and bigtable-emulator -->