AuthChain: Yield UserBackendInterface instead of UserBackend

refs #8826

library/Icinga/Authentication/AuthChain.php

@@ -6,6 +6,7 @@ namespace Icinga\Authentication;
 use Iterator;
 use Icinga\Data\ConfigObject;
 use Icinga\Authentication\User\UserBackend;
+use Icinga\Authentication\User\UserBackendInterface;
 use Icinga\Application\Config;
 use Icinga\Application\Logger;
 use Icinga\Exception\ConfigurationError;
@@ -25,7 +26,7 @@ class AuthChain implements Iterator
     /**
      * The consecutive user backend while looping
      *
-     * @var UserBackend
+     * @var UserBackendInterface
      */
     private $currentBackend;
 
@@ -53,7 +54,7 @@ class AuthChain implements Iterator
     /**
      * Return the current user backend
      *
-     * @return UserBackend
+     * @return UserBackendInterface
      */
     public function current()
     {