chore: dump maven version along with java (#1092)

Author: igorbernstein2

synthtool/gcp/templates/java_library/.kokoro/build.sh

@@ -23,8 +23,8 @@ cd ${scriptDir}/..
 # include common functions
 source ${scriptDir}/common.sh
 
-# Print out Java version
-java -version
+# Print out Maven & Java version
+mvn -version
 echo ${JOB_TYPE}
 
 # attempt to install 3 times with exponential backoff (starting with 10 seconds)