Fix Warning on PHP8

PHP8 emits a warning when `final` and `private` are used on the same method. ``` Warning: Private methods cannot be final as they are never overridden by other classes in {file} on line {line} ``` Example: https://3v4l.org/p2Ccp

Author: mgrinspan

src/Application/Application.php

@@ -16,7 +16,7 @@ protected function __construct()
  // singleton construct required this method to be protected/private
  }
 
- final private function __clone()
+ final protected function __clone()
  {
  // singleton construct required this method to be protected/private
  }