Remove a MR index with state_id, iid synchronously

What does this MR do and why?

index_merge_requests_on_target_project_id_and_iid_and_state_id index has already been removed asynchronously on GitLab.com and needs to be also ran for other installations synchronously.

Verification that the index has been removed from Gitlab.com

It would've printed out the detail of the index instead if it existed.

Database Migrations

UP

(454262-index_merge_requests_on_target_project_id_and_iid_and_state_id-synchronous-database-index ✗)  ➜  rake db:migrate                                                                  [14:38 23/04]
main: == [advisory_lock_connection] object_id: 129020, pg_backend_pid: 44450
main: == 20240423024034 DropIndexMergeRequestsOnTargetProjectIdAndIidAndStateId: migrating
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0136s
main: -- indexes(:merge_requests)
main:    -> 0.0101s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:merge_requests, {:algorithm=>:concurrently, :name=>"index_merge_requests_on_target_project_id_and_iid_and_state_id"})
main:    -> 0.0020s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20240423024034 DropIndexMergeRequestsOnTargetProjectIdAndIidAndStateId: migrated (0.0383s)

main: == [advisory_lock_connection] object_id: 129020, pg_backend_pid: 44450
ci: == [advisory_lock_connection] object_id: 129460, pg_backend_pid: 44452
ci: == 20240423024034 DropIndexMergeRequestsOnTargetProjectIdAndIidAndStateId: migrating
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0005s
ci: -- indexes(:merge_requests)
ci:    -> 0.0087s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0002s
ci: -- remove_index(:merge_requests, {:algorithm=>:concurrently, :name=>"index_merge_requests_on_target_project_id_and_iid_and_state_id"})
ci:    -> 0.0022s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0002s
ci: == 20240423024034 DropIndexMergeRequestsOnTargetProjectIdAndIidAndStateId: migrated (0.0232s)

ci: == [advisory_lock_connection] object_id: 129460, pg_backend_pid: 44452

DOWN

(454262-index_merge_requests_on_target_project_id_and_iid_and_state_id-synchronous-database-index)  ➜  rake db:migrate:down:ci VERSION=20240423024034                                     [14:37 23/04]
ci: == [advisory_lock_connection] object_id: 130000, pg_backend_pid: 43073
ci: == 20240423024034 DropIndexMergeRequestsOnTargetProjectIdAndIidAndStateId: reverting
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0129s
ci: -- index_exists?(:merge_requests, [:target_project_id, :iid, :state_id], {:name=>"index_merge_requests_on_target_project_id_and_iid_and_state_id", :algorithm=>:concurrently})
ci:    -> 0.0091s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0002s
ci: -- add_index(:merge_requests, [:target_project_id, :iid, :state_id], {:name=>"index_merge_requests_on_target_project_id_and_iid_and_state_id", :algorithm=>:concurrently})
ci:    -> 0.0023s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0003s
ci: == 20240423024034 DropIndexMergeRequestsOnTargetProjectIdAndIidAndStateId: reverted (0.0421s)

ci: == [advisory_lock_connection] object_id: 130000, pg_backend_pid: 43073

(454262-index_merge_requests_on_target_project_id_and_iid_and_state_id-synchronous-database-index ✗)  ➜  rake db:migrate:down:main VERSION=20240423024034                                 [14:38 23/04]
main: == [advisory_lock_connection] object_id: 123820, pg_backend_pid: 43749
main: == 20240423024034 DropIndexMergeRequestsOnTargetProjectIdAndIidAndStateId: reverting
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0129s
main: -- index_exists?(:merge_requests, [:target_project_id, :iid, :state_id], {:name=>"index_merge_requests_on_target_project_id_and_iid_and_state_id", :algorithm=>:concurrently})
main:    -> 0.0091s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- add_index(:merge_requests, [:target_project_id, :iid, :state_id], {:name=>"index_merge_requests_on_target_project_id_and_iid_and_state_id", :algorithm=>:concurrently})
main:    -> 0.0022s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20240423024034 DropIndexMergeRequestsOnTargetProjectIdAndIidAndStateId: reverted (0.0367s)

main: == [advisory_lock_connection] object_id: 123820, pg_backend_pid: 43749

MR acceptance checklist

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Related to #454262