Update Stitching example to .net core 3.1

[jfheins]

The example for stitching (i.e. "hotchocolate-examples/misc/Stitching/Gateway/Gateway.csproj") still uses .net core 2.1. Although this is a LTS release, it would be helpful if the example was also working with a newer .net core version. (e.g. .net core 3.1)

When I try to migrate the example to .net core 3.1, it does not work any more:

System.Net.Http.HttpRequestException: An error occurred while sending the request.
 ---> System.IO.IOException: The response ended prematurely.
   at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)

[michaelstaib]

We will have new examples coming at the end of this month.