Upgrading Uno Toolkit

Upgrading to Uno Toolkit 7.1

1. Bump to Uno 6.0

   Version 7.1 of Uno Toolkit now requires Uno 6.0.

2. Switch from ReturnType to InputReturnType

   The InputExtensions helper has been updated to use the built-in InputReturnType enum (provided by Uno v6+) instead of the legacy ReturnType enum.

   // No code changes required in XAML, but under the hood
   // the attached property now uses InputReturnType:
   public static DependencyProperty ReturnTypeProperty =
       DependencyProperty.RegisterAttached(
           "ReturnType",
           typeof(InputReturnType),
           typeof(InputExtensions),
           new PropertyMetadata(InputReturnType.Default, OnReturnTypeChanged));