Update grpc-go and protobuf

This is a pre-requisite to updating labkit in !4535 (merged).

grpc-go diff https://github.com/grpc/grpc-go/compare/v1.38.0..v1.40.0

protobuf diff: https://github.com/protocolbuffers/protobuf-go/compare/v1.26.0..v1.28.0

The change in https://github.com/grpc/grpc-go/pull/4474 caused
the status pointer to be stored rather than the encoded protobuf.
We need to use testhelper.RequireGrpcError instead of trying to
directly compare the errors.