Proj6012: Specify SDK version roll-forward policy

A global.json file should specify a valid .NET SDK version roll-forward policy. By doing so, the .NET CLI will have a predictable release when the requested SDK version is not installed.

Non-compliant

The sdk.rollForward property is missing, or its value is not a valid roll-forward policy.

Compliant

global.json

{
  "sdk": {
    "version": "10.0.400",
    "rollForward": "latestPatch"
  }
}