V14: Reintroduce umbraco api controller as obsolete

[bergmania]

Description

This PR Reintroduces the UmbracoApiController with a massive obsolete warning.

Test

• The routing should still work
• It is expected serialization is now using System.Text.Json with camel case etc.

[kjac]

I've added a few comments for you to consider - feels like something was reverted that shouldn't have been.

I also added a few more obsoletion messages here and there 😄

The code tests out nicely 💪 this fine controller:

using Microsoft.AspNetCore.Mvc;
using Umbraco.Cms.Web.Common.Controllers;

namespace Umbraco.Cms.Web.UI.Custom;

public class MyApiController : UmbracoApiController
{
    public IActionResult SayHello() => Ok("Hello!");
}

...is auto-routed as expected:

[bergmania]

FrontEndRoutes still uses the stuff you obsoleted

[kjac]

Good catch 👍 I have obsoleted that constructor too.