Fix background pipeline stages migration
What does this MR do?
This merge request fixes background migration for migrating pipeline stages by disabling Rails-specific STI mechanisms in the migration. Single table inheritance mechanism resulted in loading an invalid class when performing the background migrationo.
Does this MR meet the acceptance criteria?
- Changelog entry added, if necessary
- Tests added for this feature/bug
- Has been reviewed by Backend
What are the relevant issue numbers?