Add background migrations for filling nullified `file_location` on `ci_job_ artifacts` table

What does this MR do?

This MR adds the background migrations to fill nullified file_location column on ci_job_artifacts table.

This should be executed after legacy artifacts background migrations has done.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/50682