Migrate remaining users with null private_profile

What does this MR do?

Triggering a post deployment migration for remaining users (~90.000 users) to have their private_profile migrated from null to false.

Note: The background migration worker class is already deployed.

More context in the follow up MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29941

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

Performance 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