Evaluate Grpc.Net.Client changes for GAX impact

[jskeet]

See:

• Validate Windows version when using WinHttpHandler grpc/grpc-dotnet#2226
• Add net462 build to Grpc.Net.Client with WinHttpHandler default grpc/grpc-dotnet#2220
• Validate Windows version when using WinHttpHandler grpc/grpc-dotnet#2229

Changing from Grpc.Core to Grpc.Net.Client seamlessly is great if it works (although it could cause some surprises in terms of logging etc). It sounds like the only cases we need to worry about are ".NET Framework, on Windows Server 2022, with an API that uses client/bidi-streaming." We could potentially detect whether any APIs use client/bidi-streaming and still fall back to Grpc.Core when on .NET Framework by default for those - or do the more complex OS check if necessary. We should work out how many APIs actually have client/bidi-streaming methods.