Replace ControllerInterface with PSR-15 interface

The custom interface already had the same signature as the
one from the standard (except for the return type hint), so
why not use that one now? :)

tests/Api/Controller/ApiControllerTestCase.php

@@ -11,16 +11,16 @@
 
 namespace Flarum\Tests\Api\Controller;
 
-use Flarum\Http\Controller\ControllerInterface;
 use Flarum\Tests\Test\TestCase;
 use Flarum\User\User;
 use Illuminate\Support\Arr;
 use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Server\RequestHandlerInterface;
 
 abstract class ApiControllerTestCase extends TestCase
 {
     /**
-     * @var ControllerInterface
+     * @var RequestHandlerInterface
      */
     protected $controller;