Bump protobuf-java from 3.8.0 to 3.9.0 (#1009)

Bumps [protobuf-java](https://github.com/protocolbuffers/protobuf) from 3.8.0 to 3.9.0.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Commits](protocolbuffers/protobuf@v3.8.0...v3.9.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

exonum-java-binding/pom.xml

@@ -115,7 +115,7 @@
     <junit.version>4.12</junit.version>
     <junit.jupiter.version>5.5.0</junit.jupiter.version>
     <powermock.version>2.0.2</powermock.version>
-    <protobuf.version>3.8.0</protobuf.version>
+    <protobuf.version>3.9.0</protobuf.version>
     <mockito-core.version>3.0.0</mockito-core.version>
     <guava.version>28.0-jre</guava.version>
     <vertx.version>3.7.1</vertx.version>