Add partial index on locked merge requests

In #220771 (closed), the /api/v4/internal/allowed endpoint would timeout after 60 seconds due to this query in MatchingMergeRequest#match? (https://gitlab.com/gitlab-org/gitlab/blob/e166e5747c6e8130f1227b427113f1231d9f46f1/lib/gitlab/checks/matching_merge_request.rb#L14-17):

SELECT "merge_requests".* FROM "merge_requests" WHERE "merge_requests"."target_project_id" = 7764 AND ("merge_requests"."state_id" IN (4)) AND "merge_requests"."in_progress_merge_commit_sha" = 'db21cf25663ce1ee2a548ce49db07e8882ed86ff' AND "merge_requests"."target_branch" = 'master' LIMIT 11;

Before

 Limit  (cost=0.56..69692.37 rows=1 width=726) (actual time=69976.787..69976.787 rows=0 loops=1)
   Buffers: shared hit=2616 read=51601 dirtied=2358
   I/O Timings: read=69297.775
   ->  Index Scan using index_merge_requests_on_target_project_id_and_iid on public.merge_requests  (cost=0.56..69692.37 rows=1 width=726) (actual time=69976.784..69976.784 rows=0 loops=1)
         Index Cond: (merge_requests.target_project_id = 7764)
         Filter: ((merge_requests.state_id = 4) AND ((merge_requests.in_progress_merge_commit_sha)::text = 'db21cf25663ce1ee2a548ce49db07e8882ed86ff'::text) AND ((merge_requests.target_branch)::text = 'master'::text))
         Rows Removed by Filter: 52239
         Buffers: shared hit=2616 read=51601 dirtied=2358
         I/O Timings: read=69297.775

Time: 1.166 min
  - planning: 0.343 ms
  - execution: 1.166 min
    - I/O read: 1.155 min
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 2616 (~20.40 MiB) from the buffer pool
  - reads: 51601 (~403.10 MiB) from the OS file cache, including disk I/O
  - dirtied: 2358 (~18.40 MiB)
  - writes: 0

After

 Limit  (cost=0.28..6.73 rows=1 width=726) (actual time=0.037..0.037 rows=0 loops=1)
   Buffers: shared hit=5
   ->  Index Scan using merge_request_test_idx on public.merge_requests  (cost=0.28..6.73 rows=1 width=726) (actual time=0.035..0.035 rows=0 loops=1)
         Index Cond: (merge_requests.target_project_id = 7764)
         Filter: (((merge_requests.in_progress_merge_commit_sha)::text = 'db21cf25663ce1ee2a548ce49db07e8882ed86ff'::text) AND ((merge_requests.target_branch)::text = 'master'::text))
         Rows Removed by Filter: 3
         Buffers: shared hit=5

Time: 0.528 ms
  - planning: 0.431 ms
  - execution: 0.097 ms
    - I/O read: 0.000 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 5 (~40.00 KiB) from the buffer pool
  - reads: 0 from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

Migration timings

Duration: 12.505 min