Upgrade Maven wrapper to 3.9.4

Author: pacphi

.mvn/wrapper/maven-wrapper.properties

@@ -1,2 +1,2 @@
-distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.6.1/apache-maven-3.6.1-bin.zip
-wrapperUrl=https://repo.maven.apache.org/maven2/io/takari/maven-wrapper/0.5.5/maven-wrapper-0.5.5.jar
+distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.9.4/apache-maven-3.9.4-bin.zip
+wrapperUrl=https://repo.maven.apache.org/maven2/io/takari/maven-wrapper/0.5.6/maven-wrapper-0.5.6.jar