[flatbuffers] Fix to working with vcpkg gRPC

[youknowone]

What does your PR fix?

The last flatbuffers release working with gRPC 1.14 but not with current version. Because vcpkg only support the lastest version, flatbuffers+gRPC use case is now not working with vcpkg.

This patch temporary fix flatbuffers working with the latest gRPC.
When the next flatbuffers version is released with latest gRPC support, this patch can be removed.

Which triplets are supported/not supported? Have you updated the CI baseline?

Not changed

Does your PR follow the maintainer guide?

[BillyONeal]

Is upstream doing this and/or have they indicated support for it?

[youknowone]

I found this is an opened issue for the upstream. google/flatbuffers#5836

Actually I am not sure what will be the best fix for vcpkg. By the issue, gRPC and flatbuffers have conflict requirement, so this will be unlikely to be solved in short time.

If vcpkg flatbuffers can use gRPC 1.16.1, it will be solved but vcpkg only support single latest version. What will be the best solution for this problem?

[NancyLi1013]

Hi @youknowone

Thanks for your PR.

GRPC has been updated to 1.33.1 in vcpkg. It's impossible to downgrade it to 1.16. I also noticed that there were some discussions about the support between flatbuffers + grpc in grpc upstream grpc/grpc#20594.

So it might need more time to implement this feature on both flatbuffers and grpc sides.

For vcpkg, we recommend to use the changes or feature from upstream.

If you want to support this feature in vcpkg, you can make a pull request to flatbuffers or grpc upstream.

[youknowone]

Thank you for the review