Add post deployment migration to complete Pages metadata migration

What does this MR do?

With !17993 (merged) we added a background migration in %12.4. This MR is to complete the migration by "stealing" the remaining Sidekiq jobs.

There are no such jobs left on GitLab.com and we have 1 project that has no its data migrated (not sure why). The initial variant of this MR included the query to populate project_pages_metadata for such projects but given the time it will take to run we decided not to do this and rely on the record being created/updated when Pages are deployed for these projects (e.g. https://gitlab.com/gitlab-org/gitlab/blob/9e7db9b1f4e0726874f4dabb9db1421cac736ce3/app/services/projects/update_pages_service.rb#L54-58). See !19928 (comment 254801462) for more details.

Related to #28781 (closed).

Screenshots

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