Handling application updates with App Installer and MSIX in Windows 10 19H1

The code for checking updates programmatically is causing an app crash for me when installing my app on Windows 10 Enterprise LTSC build 1809. It works fine on other editions or even LTSC build 21H1.

I got it working using the `Microsoft.Windows.SDK.Contracts` NuGet package instead as described here (had to convert from package.config to PackageReference)

https://docs.microsoft.com/en-us/windows/apps/desktop/modernize/desktop-to-uwp-enhance#earlier-versions-of-net-install-the-microsoftwindowssdkcontracts-nuget-package

Sample code

```cs

var packageManager = new PackageManager();
var currentPackage = packageManager.FindPackageForUser(string.Empty, Package.Current.Id.FullName);
var result = await currentPackage.CheckUpdateAvailabilityAsync();

if (result.ExtendedError != null) {
     LogError("Update availability error", result.ExtendedError);
}

var hasUpdate = (result.Availability == PackageUpdateAvailability.Available);

if (hasUpdate) {

   // there is an update available

}

```