Bump Gradle in generated Java projects to 6.7.1 (#8079)

* Bump Gradle in Java projects from 6.0.1 to 6.7.1

Regular maintenance, 6.0.1 was released November 18th, 2019 and the latest release (November 16th, 2020) is 6.7.1. This updates all the wrapper templatesto the latest version.

* Updated generated Java samples

Generated via `./bin/generate-samples.sh bin/configs/java-*`

* update samples

Co-authored-by: William Cheng <wing328hk@gmail.com>

samples/client/petstore/java/okhttp-gson/gradlew

@@ -82,6 +82,7 @@ esac
 
 CLASSPATH=$APP_HOME/gradle/wrapper/gradle-wrapper.jar
 
+
 # Determine the Java command to use to start the JVM.
 if [ -n "$JAVA_HOME" ] ; then
     if [ -x "$JAVA_HOME/jre/sh/java" ] ; then
@@ -129,6 +130,7 @@ fi
 if [ "$cygwin" = "true" -o "$msys" = "true" ] ; then
     APP_HOME=`cygpath --path --mixed "$APP_HOME"`
     CLASSPATH=`cygpath --path --mixed "$CLASSPATH"`
+
     JAVACMD=`cygpath --unix "$JAVACMD"`
 
     # We build the pattern for arguments to be converted via cygpath