Limit project download limit to group namespaces

Resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/150

What does this MR do and why?

Context: Project downloads limiting can only be configured for top-level group namespaces with an Ultimate plan.

This MR updates Users::Abuse::ProjectsDownloadBanCheckService to return early (with success response–download can proceed) when a user downloads a project under a user namespace.

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