Fix background migration temporary class definition

What does this MR do?

The intention of the temporary class is to decouple the background migration code from the codebase. This helps avoid the need to update the background migration and its tests due to changes in the codebase, and also ensures the background migration jobs can run on different versions of the app.

But currently, the perform method is using the codebase's PackageFile class. This MR fixes that mistake.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry => none needed
• 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.