Fix orphaned promoted issues

What does this MR do?

Migration to populate promoted issues missing promoted_to_epic_id field.

There are 411 promoted issues to epic with nil promoted_to_epic_id, the problem here is that the only way to find those issues is through a system note which contains the epic_iid for example: promoted to epic &99

This merge request schedules a background migration using temporary indexes:

there are only 3 orphan issues in the context where these queries ran

Scheduling queries

SELECT "notes"."id" 
FROM   "notes" 
WHERE  "notes"."noteable_type" = $1 
       AND "notes"."system" = $2 
       AND ( notes.note LIKE 'promoted to epic%' ) 
ORDER  BY "notes"."id" ASC 
LIMIT  $3 

SELECT "notes"."id" 
FROM   "notes" 
WHERE  "notes"."noteable_type" = $1 
       AND "notes"."system" = $2 
       AND ( notes.note LIKE 'promoted to epic%' ) 
       AND "notes"."id" >= 1 
ORDER  BY "notes"."id" ASC 
LIMIT  $3 offset $4 


SELECT Min(id), 
       Max(id) 
FROM   "notes" 
WHERE  "notes"."noteable_type" = $1 
       AND "notes"."system" = $2 
       AND ( notes.note LIKE 'promoted to epic%' ) 
       AND "notes"."id" >= 1 

Update query on background migration

WITH promotion_notes AS (
  SELECT noteable_id, note as promotion_note, projects.namespace_id as epic_group_id FROM notes
  INNER JOIN projects ON notes.project_id = projects.id
  WHERE notes.noteable_type = 'Issue' AND notes.system = true AND notes.note like 'promoted to epic%'
  AND notes.id BETWEEN 1 AND 3
), promoted_epics AS (
  SELECT epics.id as promoted_epic_id, promotion_notes.noteable_id as issue_id FROM epics
  INNER JOIN promotion_notes on epics.group_id = promotion_notes.epic_group_id
  WHERE concat('promoted to epic &', epics.iid) = promotion_notes.promotion_note
)
UPDATE issues
SET promoted_to_epic_id = promoted_epic_id
FROM promoted_epics
WHERE issues.id = promoted_epics.issue_id
AND issues.promoted_to_epic_id IS NULL

Related to #194177 (closed)