`BulkInsert` cop to also check top-level namespace use of `::Gitlab`

What does this MR do?

The Gitlab/BulkInsert cop previously allowed uses of Gitlab::Database.bulk_insert when the Gitlab module was accessed via the top-level namespace ::Gitlab.

Related issues:

• #207989 (closed)
• #221032

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done