Fix sign-up status icon in the admin dashboard

What does this MR do?

As reported in #66499 (moved), the sign-up status icon in the admin dashboard is always displayed as a green dot regardless of the actual sign-up feature status.

This was introduced by a6ac0a27 and its cause is a missing call to allow_signup? to check the actual status.

This MR fixes the issue by adding the missing call.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content