Update to protobuf 3.9.1 (#766)

Author: elharo

pom.xml

@@ -531,7 +531,7 @@
     <project.jsr305.version>3.0.2</project.jsr305.version>
     <project.gson.version>2.8.5</project.gson.version>
     <project.jackson-core2.version>2.9.9</project.jackson-core2.version>
-    <project.protobuf-java.version>3.6.1</project.protobuf-java.version>
+    <project.protobuf-java.version>3.9.1</project.protobuf-java.version>
     <project.guava.version>28.0-android</project.guava.version>
     <project.xpp3.version>1.1.4c</project.xpp3.version>
     <project.commons-logging.version>1.2</project.commons-logging.version>