Fixer/Analyzer: Remove UseEndpoints() and hoist route registrations top level to WebApplication

[davidfowl]

With the new hosting model, it's possible to register routes top level instead of using UseEndpoints. To help customer learn about this, we can have an fixer that can hoist the route registrations to hang off WebApplicatin:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.UseRouting();
app.UseEndpoints(endpoints =>
{
    endpoints.MapGet("/", () => "Hello World!");
});

We would suggest this:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.UseRouting();
app.MapGet("/", () => "Hello World!");

app.Run();

Stretch goal is to also remove UseRouting but we would need to only remove it if it wasn't intentionally put top level (like set to re-run middleware).

[BrennanConroy]

UseEndpoints fixer should only apply if it's the last middleware called

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.