Use work_item labels when migration finished

What does this MR do and why?

This introduces the MigrationStatus for checking if a migration has been finished or not. Instead of waiting until a background migration has been finalized, we can already change our queries when the background migration has been finished.

Especially for label_links we gain performance as we only need to read the labels of a work item instead of performing a UNION query also for its legacy epic label_link.

This MR changes the BulkResolver and LabelFilter to start reading only from the work_item labels when the migration has been completed. Other changes to read the label_links only from the work item will follow.

• Before: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/46392/commands/141530
• After: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/46392/commands/141533

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable work_item_planning_view feature flag
2. Enable use_namespace_traversal_ids_for_work_items_finder feature flag
3. Enable the Performance bar
4. Go to a Group -> Work Items
5. Filter the work items based on a label
6. Check the GetWorkItemsFullEE or GetWorkItemsSlimEE` query

Before this MR: You'd see that the query for filtering labels is a UNION query

AND EXISTS (
SELECT "label_links".*
FROM (
    (
      SELECT "label_links"."label_id"
      FROM "label_links"
      WHERE "label_links"."target_type" = 'Issue'
        AND "label_links"."target_id" = "issues"."id"
        AND "label_links"."label_id" = 10690700
    )
    UNION ALL
    (
      SELECT "label_links"."label_id"
      FROM "label_links"
      WHERE "label_links"."target_type" = 'Epic'
        AND "label_links"."target_id" = (
          SELECT "epics"."id"
          FROM "epics"
          WHERE "epics"."issue_id" = "issues"."id"
        )
        AND "label_links"."label_id" = 10690700
    )
  ) label_links
)

With this MR

First ensure that the background job finished (status = 3):

select id, status from batched_background_migrations where job_class_name = 'MigrateEpicLabelLinksToWorkItems';
 id | status
----+--------
 22 |      3

Then you see on the query that it no longer performs the UNION query as it looks like:

 WHERE "label_links"."target_type" = 'Issue' AND "label_links"."target_id" = "issues"."id" AND "label_links"."label_id" = 111)

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