Block using stable .NET versions with Preview SDK

It's no surprise that Preview SDKs are unsupported. This means that we don't offer a high-confidence level publishing production apps. We should make `dotnet publish` of a stable version number (basically < `n`) to be opt-in. This warning will provide a clear indicator that that the Preview SDK is not supported for any production activities.

We could do the same thing with `dotnet build` but that seems over-the-top.

The associated error should give users guidance on how to modify their project or environment in order to proceed. It would also be useful if there was a way to opt-in to all stable artifacts being coerced to latest TFM w/o project changes if people want to test that.

We currently rely on a warning for this, but it is (A) annoying, and (B) doesn't send a strong enough message. It's the worst of all options.