Update protobuf-java

[cartermckinnon]

This project currently uses a fairly-old version of protobuf-java (3.9.1). That version was released on August 6, 2019. The current version is 3.17.3. Please update the version of protobuf-java to prevent runtime conflicts in projects which use newer versions. Enabling dependabot may be a good idea ;)

Like guava, protobuf-java is a very common dependency in Java applications these days. I think the ideal approach would be for java-nats-streaming to shade its desired version of protobuf so as to not conflict with any version pulled-in by an end-user or another one of their dependencies.