Add ReturnTypeWillChange attribute to silence PHP 8.1 deprecations

[mbabker]

PHP 8.1 adds tentative return types to a number of API symbols as noted in https://wiki.php.net/rfc/internal_method_return_types which will cause deprecation notices to be emitted if return types are missing. Those can be silenced by adding the return types or adding the #[ReturnTypeWillChange] attribute until types can be added.

This PR adds the attribute (which is B/C with PHP 7 and older since the attribute syntax will be parsed as comments) to methods that will cause deprecations to be triggered, silencing these deprecation notices when running on PHP 8.1:

Return type of Ratchet\Wamp\Topic::getIterator() should either be compatible with IteratorAggregate::getIterator(): Traversable, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

Return type of Ratchet\Wamp\Topic::count() should either be compatible with Countable::count(): int, or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice