Add index on merge_requests.state

What does this MR do?

Add index on merge_requests.state.

Example query - when visiting http://localhost:3000/gitlab-org/gitlab-test -:

SELECT COUNT(*) FROM "merge_requests" WHERE "merge_requests"."deleted_at" IS NULL AND "merge_requests"."target_project_id" = 1 AND ("merge_requests"."state" IN ('opened','reopened'));

Before the index - https://explain.depesz.com/s/EocWE:

                                                         QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=135085.87..135085.88 rows=1 width=8) (actual time=487.182..487.182 rows=1 loops=1)
   ->  Seq Scan on merge_requests  (cost=0.00..135085.87 rows=1 width=0) (actual time=0.320..487.174 rows=9 loops=1)
         Filter: ((deleted_at IS NULL) AND ((state)::text = ANY ('{opened,reopened}'::text[])) AND (target_project_id = 1))
         Rows Removed by Filter: 4006451
 Planning time: 0.315 ms
 Execution time: 487.221 ms
(6 rows)

After - https://explain.depesz.com/s/yl3q:

                                                                     QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=8.89..8.90 rows=1 width=8) (actual time=0.255..0.255 rows=1 loops=1)
   ->  Index Scan using index_merge_requests_on_state on merge_requests  (cost=0.43..8.89 rows=1 width=0) (actual time=0.203..0.251 rows=9 loops=1)
         Index Cond: ((state)::text = ANY ('{opened,reopened}'::text[]))
         Filter: ((deleted_at IS NULL) AND (target_project_id = 1))
         Rows Removed by Filter: 31
 Planning time: 0.431 ms
 Execution time: 0.358 ms

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?