Consider not setting PreserveCompilationContext in the RazorSDK when targeting 6.0 or later

[pranavkm]

Related issue: https://github.com/dotnet/cli/issues/9135. We set PreserveCompilationContext=true for all project types in the Razor.Sdk. This results in an refs directory to be created in the build output directory which is undesirable. We should consider setting PreserveCompilationContext=true only if the app's an executable.

[pranavkm]

We might not need to do this if we get this feature - dotnet/sdk#2122 in the Sdk. We could leave PreserveCompilationContext untouched and conditionally set the new property instead.

[mkArtakMSFT]

Moved to 2.2.0-preview1 for tracking purposes only. We most probably will get the referenced SDK feature and would close this then.