Add requires_admin meta for feature flag in multiple specs

What does this MR do?

Adds :requires_admin meta in multiple specs to facilitate enabling :invite_members_group_modal feature flag

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content