Add ReturnTypeWillChange to extended reflection classes

[derrabus]

PHP 8.1 will introduce a process to add return types to internal methods, see https://wiki.php.net/rfc/internal_method_return_types

This library extends ReflectionProperty and overrides public methods that are affected by this change. PHP 8.1 triggers deprecation warnings unless we declare that a future version of this library will add these return types as well. We can do so by adding the ReturnTypeWillChange attribute to those methods.

This PR suggests to do just that because adding the actual return types would break code that extends our classes. As there's already a 3.0.x branch open, I suggest to add the return types once this PR has been merged up.

[greg0ire]

Thanks @derrabus !