Set visibility level 'Private' for restricted Internal imported projects

What does this MR do?

Sets visibility level to Private for projects that are being imported with Internal visibility level, when that level is being restricted in admin settings. This is done to minimise the potential risk of gitlab-ee#12388 going live and breaking internal project imports.

Potential risk would be unintentional setting of visibility level to Private when visibility setting is not Internal or it is not restricted in admin settings.

Steps to reproduce

1. Go to admin settings, restrict Internal visibility level.
2. As admin, create new Internal project (restrictions above do not apply to admins)
3. Export created project
4. Impersonate a user and import previously exported project
5. Once export is complete, verify Project visibility level is set to Internal

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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