Add deprecation notice for domain in directory sync
Description
This paves the way for @mattgd's PR https://github.com/workos/workos-php/pull/215, which will remove the $domain parameter in the next major release. Since the domain is no longer used today, this provides a graceful deprecation notice for developers to begin the transition to alternative methods.
I also caught a bug on DirectorySync.php:105 that referenced $group instead of $user. Group is not a defined parameter.
This PR relies on the testing methods introduced in https://github.com/workos/workos-php/pull/272. Please merge #272 first.
Documentation
Does this require changes to the WorkOS Docs? E.g. the API Reference or code snippets need updates.
[ ] Yes
If yes, link a related docs PR and add a docs maintainer as a reviewer. Their approval is required.