Requeue MigrateProjectAuthorizations batched migration

What does this MR do and why?

Requeues the MigrateProjectAuthorizations batched background migration. It backfills project_authorizations_for_migration from project_authorizations as part of restoring the unique index on project_authorizations (https://gitlab.com/gitlab-org/gitlab/-/issues/418205).

The first run finished, but a bug in the sync trigger removed rows from project_authorizations_for_migration while it ran. At least one row is missing on GitLab.com. The trigger is fixed in !246505 (merged), which has merged. This MR re-runs the backfill to restore the rows that were lost.

The job code is unchanged. It inserts with ON CONFLICT DO NOTHING, so the re-run only adds missing rows and leaves existing ones alone. Following the re-queue guidelines (https://docs.gitlab.com/development/database/batched_background_migrations/#re-queue-batched-background-migrations), the original queue migration becomes a no-op and the migration dictionary points to the new queue migration.

The trigger fix must run before the backfill. Both are post-deployment migrations and their timestamps order them correctly: the fix is 20260720104911, the requeue is 20260810120000.

References

• https://gitlab.com/gitlab-org/gitlab/-/issues/525999
• Missing row investigation: https://gitlab.com/gitlab-org/gitlab/-/work_items/526000#note_3527316319
• Trigger fix (merged): !246505 (merged)
• Overall effort: https://gitlab.com/gitlab-org/gitlab/-/issues/418205

MR acceptance checklist

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Related to https://gitlab.com/gitlab-org/gitlab/-/issues/525999