Add background migration to fill file stores from `NULL` to `1`

What does this MR do?

This MR adds background migrations to fill file stores for

• ci_job_artifacts.file_store
• lfs_objects.file_store
• uploads.store

The legacy artifacts are NOT targeted

• ci_builds.artifacts_file_store
• ci_builds.artifacts_metadata_store

Because we're migrating legacy artifacts to new artifacts (ci_job_artifacts.file_store), and in the migration process, the legacy file store columns will be filled by a value. https://gitlab.com/gitlab-org/gitlab-ee/issues/4172

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18612 adds a validation (AR or DB unique constraint) to prevent further NULL file store.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/45337 Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/45476