Warning on cross-targeting WPF app using Microsoft.NET.Sdk.WindowsDesktop

[ericstj]

Create a netcoreapp3.1 WPF application. Make it also target net5.0.

<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">

  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFrameworks>netcoreapp3.1;net5.0-windows</TargetFrameworks>
    <UseWPF>true</UseWPF>
  </PropertyGroup>

</Project>

Expect: build clean

Actual: warning:

warning NETSDK1137: It is no longer necessary to use the Microsoft.NET.Sdk.WindowsDesktop SDK. Microsoft.NET.Sdk can be used instead.

This warning only applies to net5.0, changing the SDK will cause this project to fail to build on netcoreapp3.1. Introduced in #12386 cc @sfoslund @dsplaisted

[dsplaisted]

This is fixed by #13441 (which also lets you use Microsoft.NET.Sdk for .NET Core 3.x WPF projects)