Support the use of PHP 8.0 "mixed" type

Fix failing static tests

Author: ihor-sviziev

lib/internal/Magento/Framework/ObjectManager/Code/Generator/Proxy.php

@@ -1,30 +1,23 @@
 <?php
 /**
- * Proxy generator
- *
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
 declare(strict_types=1);
 
 namespace Magento\Framework\ObjectManager\Code\Generator;
 
-/**
- * Class Proxy
- *
- * @package Magento\Framework\ObjectManager\Code\Generator
- */
 class Proxy extends \Magento\Framework\Code\Generator\EntityAbstract
 {
     /**
      * Entity type
      */
-    const ENTITY_TYPE = 'proxy';
+    public const ENTITY_TYPE = 'proxy';
 
     /**
      * Marker interface
      */
-    const NON_INTERCEPTABLE_INTERFACE = \Magento\Framework\ObjectManager\NoninterceptableInterface::class;
+    public const NON_INTERCEPTABLE_INTERFACE = \Magento\Framework\ObjectManager\NoninterceptableInterface::class;
 
     /**
      * Returns default result class name