Bump protobuf-java from 3.0.0 to 3.15.6

Bumps [protobuf-java](https://github.com/protocolbuffers/protobuf) from 3.0.0 to 3.15.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.0.0...v3.15.6)

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

molten-dependencies/pom.xml

@@ -31,7 +31,7 @@
     <vertx.version>3.9.4</vertx.version>
     <netty.version>4.1.59.Final</netty.version>
     <reactor-netty.version>1.0.4</reactor-netty.version>
-    <protobuf.version>3.0.0</protobuf.version>
+    <protobuf.version>3.15.6</protobuf.version>
     <retrofit.version>2.9.0</retrofit.version>
     <okhttp.version>4.9.1</okhttp.version>
   </properties>