NETSDK1137 should occur even when targeting .NET Core 3.x or .NET Framework

[na1307]

https://github.com/dotnet/sdk/blob/main/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.targets#L1444-L1451

According to this code, WindowsDesktop will be loaded not only when targeting .NET 5 or higher, but also when targeting .NET Core 3.x or .NET Framework. So you don't need to use Microsoft.NET.Sdk.WindowsDesktop no matter which framework you target.

https://github.com/dotnet/sdk/blob/main/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.DefaultItems.targets#L145-L149

However, the current behavior only raises a warning when targeting .NET 5 or later. I think this warning should be raised regardless of which framework is targeted.