Update .gitignore so that .jar file isn't included (#7505)

Issue #7503 was raised about including maven-wrapper.jar. @ludoch helpfully pointed out https://maven.apache.org/wrapper/; effectively we can remove the .jar, it makes using the sample a little more difficult, but with the rule to not include binary artifacts into our source code, seems like the right thing to do

Author: iennae

appengine-java8/springboot-helloworld/.gitignore

@@ -1,5 +1,5 @@
 target/
-!.mvn/wrapper/maven-wrapper.jar
+.mvn/wrapper/maven-wrapper.jar
 
 ### STS ###
 .apt_generated
@@ -21,4 +21,4 @@ build/
 nbbuild/
 dist/
 nbdist/
-.nb-gradle/
+.nb-gradle/