drop App generic for UsePrism method

[dansiegel]

Description

When Prism for .NET MAUI began we anticipated requiring at least some code in the Application class and continuing to have a PrismApplication like we do with Prism for WPF, Uno, & Xamarin.Forms. As time has gone on and the .NET MAUI API has taken shape, and as Prism for .NET MAUI has grown to work with their updated API, there is no longer a need for a PrismApplication base class. As a result we really do not need to provide this as part of the bootstrapping for Prism.

BREAKING CHANGE

// Old Code
var builder = MauiApp.CreateBuilder();
builder.UsePrismApp<App>(prism => {
    // configure Prism
});

This will now change to:

var builder = MauiApp.CreateBuilder();
builder.UseMauiApp<App>()
    .UsePrism(prism => {
        // Configure Prism
    });