Update projects and remove WindowsDesktop SDK reference for .NET 5

There are projects in the repo that target the old WindowsDesktop SDK and don't compile in .NET 5. These should be upgraded.

```
C:\code\Work\dotnet\samples [master ≡ +2 ~0 -0 !]> dir *.csproj -r | select-string "Microsoft.NET.Sdk.WindowsDesktop"
async\async-and-await\cs\ParallelAsyncExample\ParallelAsyncExample.csproj:1:
async\async-and-await\cs\SerialAsyncExample\SerialAsyncExample.csproj:1:async\async-and-await\cs\SyncExample\SyncExample.csproj:1:
core\interop\comwrappers\IDispatch\ComWrappersIDispatch.csproj:1:
csharp\parallel\Raytracer\Raytracer.csproj:1:
csharp\parallel\Raytracer.Wpf\Raytracer.Wpf.csproj:1:
windowsforms\datagridview\CSWinFormDataGridView\CSWinFormDataGridView.csproj:1:
windowsforms\FlexGridShowcaseDemo\cs\FlexGridShowcaseDemo.csproj:1:
windowsforms\formatting-utility\cs\FormatUtility.csproj:1:
windowsforms\graphics\WinFormGraphics\WinFormGraphics.csproj:1:
windowsforms\helloworld-links\NetCore3App\NetCore3App.csproj:1:
windowsforms\helloworld-sharedsource\CoreWindowsFormsApp\CoreWindowsFormsApp.csproj:1:
windowsforms\MSIX-WindowsForms\CoreWinFormsApp1\CoreWinFormsApp1.csproj:1:
wpf\HelloWorld-WithLinkedFiles\HelloWorldCore\HelloWorldCore.csproj:1:
wpf\WPF-WinRT\WPF-WinRT-Core\WPF-WinRT-Core.csproj:1:

C:\code\Work\dotnet\samples [master ≡ +2 ~0 -0 !]> dir *.vbproj -r | select-string "Microsoft.NET.Sdk.WindowsDesktop"
async\async-and-await\vb\ParallelAsyncExample\ParallelAsyncExample.vbproj:1:
async\async-and-await\vb\SerialAsyncExample\SerialAsyncExample.vbproj:1:
async\async-and-await\vb\SyncExample\SyncExample.vbproj:1:
windowsforms\Conway's-Game-of-Life\VB\Conway's-Game-of-Life\Conway's-Game-of-Life.vbproj:1:
windowsforms\formatting-utility\vb\FormatUtility.vbproj:1:
windowsforms\graphics\WinFormGraphics_vb\winformgraphics_vb.vbproj:1:
windowsforms\Sudoku\VB\sudoku\sudoku.vbproj:1:
windowsforms\TaskDialogDemo\vb\TaskDialogDemo.vbproj:1:
```