V14: Reintroduce umbraco api controller as obsolete (#16263)

* Revert "v14: Remove mentions of UmbracoApiController (#15863)"

This reverts commit 30e2dea57a.

* Obsolete UmbracoApiController

* Added a few more obsoletion messages

* Removed some of the reintroduced stuff again

* Add obsoletion to FrontEndRoutes controller

---------

Co-authored-by: kjac <kja@umbraco.dk>

src/Umbraco.Web.Common/Controllers/UmbracoApiControllerBase.cs

@@ -0,0 +1,27 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using Umbraco.Cms.Core.Features;
+using Umbraco.Cms.Web.Common.Attributes;
+using Umbraco.Cms.Web.Common.Authorization;
+
+namespace Umbraco.Cms.Web.Common.Controllers;
+
+/// <summary>
+///     Provides a base class for Umbraco API controllers.
+/// </summary>
+/// <remarks>
+///     <para>These controllers are NOT auto-routed.</para>
+///     <para>The base class is <see cref="ControllerBase" /> which are netcore API controllers without any view support</para>
+/// </remarks>
+[Authorize(Policy = AuthorizationPolicies.UmbracoFeatureEnabled)]
+[UmbracoApiController]
+[Obsolete("This will be removed in Umbraco 15.")]
+public abstract class UmbracoApiControllerBase : ControllerBase, IUmbracoFeature
+{
+    /// <summary>
+    ///     Initializes a new instance of the <see cref="UmbracoApiControllerBase" /> class.
+    /// </summary>
+    protected UmbracoApiControllerBase()
+    {
+    }
+}