Fix epics search query

What does this MR do?

Fix epics search query when :attempt_group_search_optimizations is enabled.

We have to use the search CTE by last in the query otherwise it tries to group by a column that is not included in the select.

Old query

WITH "epics" AS ( 
( 
       SELECT "epics".* 
       FROM   "epics" 
       WHERE  "epics"."group_id" = 9979)
) 
SELECT     "epics".* 
FROM       epics 
INNER JOIN "label_links" 
ON         "label_links"."target_id" = "epics"."id" 
AND        "label_links"."target_type" = 'Epic' 
INNER JOIN "labels" 
ON         "labels"."id" = "label_links"."label_id" 
WHERE      ( 
                      "epics"."title" ilike '%python%' 
           OR         "epics"."description" ilike '%python%') 
AND        "epics"."state_id" = 1 
AND        "labels"."title" IN ('group::portfolio management', 
                                'devops::plan') 
GROUP BY   "epics"."id" 
HAVING     ( 
                      count(DISTINCT labels.title) = 2) 
ORDER BY   "epics"."updated_at" DESC, 
           "epics"."id" DESC limit 20 offset 0

New query

WITH "epics" AS ( 
( 
           SELECT     "epics".* 
           FROM       "epics" 
           INNER JOIN "label_links" 
           ON         "label_links"."target_id" = "epics"."id" 
           AND        "label_links"."target_type" = 'Epic' 
           INNER JOIN "labels" 
           ON         "labels"."id" = "label_links"."label_id" 
           WHERE      "epics"."group_id" = 3 
           AND        "labels"."title" IN ('label1', 
                                           'label2') 
           GROUP BY   "epics"."id" 
           HAVING     ( 
                                 Count(DISTINCT labels.title) = 2))) 
SELECT   "epics".* 
FROM     epics 
WHERE    ( 
                  "epics"."title" ilike '%filtered%' 
         OR       "epics"."description" ilike '%filtered%') 
ORDER BY "epics"."id" DESC

• We can ignore the changelog since this change effect is behind a feature flag.

related to #198156 (closed)