Exclude S3216 from .NET Core

Rule S3216 (“ConfigureAwait(false)” should be used) is not relevant to .NET Core, only .NET Full. Thus, we are currently forced to create different quality profiles for .NET Core and .NET Full, and always properly assign projects to them (which is difficult to do perfectly). It would be nice not to have to do this, and instead have rules like this excluded for .NET Core.

thanks @esauser . I confirm that .NET Core does not have a SynchronizationContext anymore (for readers, this blog post has more details)

I opened #2588 to track this