QuickActions::TargetService should always be called with a container argument

In Fix members autocomplete sources for group leve... (!140139 - merged) we changed QuickActions::TargetService to use the BaseContainerService, but there are still some places where the service might be called with a project and a group (passed as an argument) as in https://gitlab.com/gitlab-org/gitlab/-/blob/ebf0e356e6c3996fbab93fe5c72cdd4e523f2e89/app/controllers/concerns/preview_markdown.rb#L8

We should change the preview_markdown concern to always pass the container in the container argument of the service.

Doing this one as a follow-up to keep the MRs smaller