Prevent namespace creation on enabled BSO and on user limit

What does this MR do and why?

This change introduces seat management controls to prevent users from creating groups and projects when block seat overages is activated and no seats are available.

The user interface text was also updated to clarify that the "Restricted access" setting now prevents both exceeding the licensed seat count and stops non-members from creating groups and projects when seats aren't available.

This helps organizations better control their GitLab usage and ensures they don't accidentally exceed their licensed capacity through new group and project creation.

References

• Resolves Prevent namespace creation on enabled BSO and o... (#576380)

Screenshots or screen recordings

When BSO is enabled, user limit is reached and general setting Allow users with up to Guest role to create groups and personal projects is activated, we now still deny the creation of groups and projects for non-billable users.

	Before	After
http://localhost:3000/
http://localhost:3000/dashboard/projects
http://localhost:3000/dashboard/groups

How to set up and validate locally

1. Activate Self-Managed BSO
2. Ensure all seats are taken, for example, by using an Ultimate license with only one seat
3. Activate general setting Allow users to Guest role to create groups and personal projects here
4. Log in as a user who has only a guest role
5. Try to create a group or project in your own user namespace
6. It should not be possible any longer, and with that, the user can't become billable 🎉

MR acceptance checklist

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