Fix PHP 8.1/8.2 deprecations

In https://tracker.phpbb.com/browse/PHPBB3-17097 some deprecations were fixed .

There are some more, which should also be fixed, maybe with a more systematic approach. That is, look where the following interfaces are implemented:

• \ArrayAccess
• \ArrayIterator
• \IteratorAggregate
• \Countable
• ...

phpbb/extension/manager.php is missing the property protected $filesystem;