[Part 2] Add and backfill resource_iteration_events group

What does this MR do and why?

DB review

Query plans

UPDATE sub_batch 100 records

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/37974/commands/116242

UPDATE 
  "resource_iteration_events" 
SET 
  "namespace_id" = "sprints"."group_id" 
FROM 
  "sprints" 
WHERE 
  "resource_iteration_events"."iteration_id" = "sprints"."id" 
  AND "resource_iteration_events"."id" IN (
    SELECT 
      "resource_iteration_events"."id" 
    FROM 
      "resource_iteration_events" 
    WHERE 
      "resource_iteration_events"."id" BETWEEN 8 
      AND 13761 
      AND "resource_iteration_events"."id" >= 8 
      AND "resource_iteration_events"."id" < 131
  )

Migration output

UP

main: == [advisory_lock_connection] object_id: 136920, pg_backend_pid: 2142
main: == 20250409172701 QueueBackfillResourceIterationEventsNamespaceId: migrating ==
main: == 20250409172701 QueueBackfillResourceIterationEventsNamespaceId: migrated (0.0433s) 

main: == [advisory_lock_connection] object_id: 136920, pg_backend_pid: 2142
ci: == [advisory_lock_connection] object_id: 137280, pg_backend_pid: 2145
ci: == 20250409172701 QueueBackfillResourceIterationEventsNamespaceId: migrating ==
ci: -- The migration is skipped since it modifies the schemas: [:gitlab_main].
ci: -- This database can only apply migrations in one of the following schemas: [:gitlab_ci, :gitlab_ci_cell_local, :gitlab_internal, :gitlab_shared].
ci: == 20250409172701 QueueBackfillResourceIterationEventsNamespaceId: migrated (0.0081s) 

ci: == [advisory_lock_connection] object_id: 137280, pg_backend_pid: 2145

DOWN

main: == [advisory_lock_connection] object_id: 133720, pg_backend_pid: 1905
main: == 20250409172701 QueueBackfillResourceIterationEventsNamespaceId: reverting ==
main: == 20250409172701 QueueBackfillResourceIterationEventsNamespaceId: reverted (0.0327s)

ci: == [advisory_lock_connection] object_id: 134480, pg_backend_pid: 1924
ci: == 20250409172701 QueueBackfillResourceIterationEventsNamespaceId: reverting ==
ci: -- The migration is skipped since it modifies the schemas: [:gitlab_main].
ci: -- This database can only apply migrations in one of the following schemas: [:gitlab_ci, :gitlab_ci_cell_local, :gitlab_internal, :gitlab_shared].
ci: == 20250409172701 QueueBackfillResourceIterationEventsNamespaceId: reverted (0.0125s)

MR acceptance checklist

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Related to #514595 (closed)