LFT-1285 - Suppress sync generator in .NET Core build

[j3parker]

We don't need the sync implementations but we especially don't want the obligations to implement sync APIs in .NET Core. The spirit of the generator is to make migrating to sync in .NET Framework sustainable, the idea generally is that in .NET Core we won't rely on sync APIs.

[j3parker]

Here's ILSpy showing the net60 build (top) without sync overloads in IPublicKeyDataProvider, and the net472 build (below) with them.