Need to upgrade the related gRPC dependencies.

[zxh]

Hi, I have met some grpc errors when I using dgraph4j: 2.0.2 and protobuf-java: 3.7.1.

The error:

Caused by: io.grpc.StatusRuntimeException: UNAVAILABLE: Connection closed while performing protocol negotiation for [HttpClientCodec#0, HttpProxyHandler#0, ProtocolNegotiators$WaitUntilActiveHandler#0, WriteBufferingAndExceptionHandler#0, DefaultChannelPipeline$TailContext#0]

And the solution is to exclude the old version of grpc dependencies in dgraph4j and added the new versions. For dgraph4j: 2.0.2 version, we could do this at pom.xml:

...
    <dependency>
      <groupId>com.google.protobuf</groupId>
      <artifactId>protobuf-java</artifactId>
      <version>3.11.1</version>
    </dependency>

    <dependency>
      <groupId>io.dgraph</groupId>
      <artifactId>dgraph4j</artifactId>
      <version>2.0.2</version>
      <exclusions>
        <exclusion>
          <groupId>io.grpc</groupId>
          <artifactId>grpc-protobuf</artifactId>
        </exclusion>
        <exclusion>
          <groupId>io.grpc</groupId>
          <artifactId>grpc-stub</artifactId>
        </exclusion>
        <exclusion>
          <groupId>io.grpc</groupId>
          <artifactId>grpc-netty</artifactId>
        </exclusion>
      </exclusions>
    </dependency>

    <dependency>
      <groupId>io.grpc</groupId>
      <artifactId>grpc-protobuf</artifactId>
      <version>1.26.0</version>
    </dependency>
    <dependency>
      <groupId>io.grpc</groupId>
      <artifactId>grpc-stub</artifactId>
      <version>1.26.0</version>
    </dependency>
    <dependency>
      <groupId>io.grpc</groupId>
      <artifactId>grpc-netty</artifactId>
      <version>1.26.0</version>
    </dependency>

...

The detail info about the issue: https://discuss.dgraph.io/t/dgraph4j-using-java-client-connect-to-dgraph-alpha-failed-docker-on-aws-ec2/5829

I think we could upgrade the related dependencies in dgrap4j to avoid this same problems.

Thanks.