Downgrade gRPC and protobuf dependencies

[jrhee17]

Motivation:

protocolbuffers/protobuf#11393

        Caused by:
        java.lang.NoSuchMethodError: java.nio.ByteBuffer.position(I)Ljava/nio/ByteBuffer;
            at com.google.protobuf.CodedOutputStream$UnsafeDirectNioEncoder.writeStringNoTag(CodedOutputStream.java:2153)
            at com.google.protobuf.CodedOutputStream$UnsafeDirectNioEncoder.writeString(CodedOutputStream.java:1922)
            at com.google.protobuf.GeneratedMessageV3.writeString(GeneratedMessageV3.java:3195)

Modifications:

• Downgrade protobuf and gRPC

Result:

• Users no longer encounter an exception when using protobuf, grpc with jre 8

[jrhee17]

Although I think this PR is ready, I would like to run the CI one more time after #4661 is merged 🙇

[ikhoon]

👍

[ikhoon]

Re-ran the failed Java 8 test.

[jrhee17]

It seems like gax-grpc was unintentionally resolving protobuf to a more recent version.
We may consider more stricter dependency resolution failure strategies (if there is an ability to limit to certain artifacts)

[minwoox]

Thanks!