Remove MR opened state index synchronously

What does this MR do and why?

idx_merge_requests_on_target_project_id_and_iid_opened 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

(455498-idx_merge_requests_on_target_project_id_and_iid_opened-synchronous-database-index-removal ✗)  ➜  rake db:migrate
main: == [advisory_lock_connection] object_id: 124200, pg_backend_pid: 38214
main: == 20240423020601 RemoveIdxMergeRequestsOnTargetProjectIdAndIidOpened: migrating
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0142s
main: -- indexes(:merge_requests)
main:    -> 0.0109s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- remove_index(:merge_requests, {:algorithm=>:concurrently, :name=>"idx_merge_requests_on_target_project_id_and_iid_opened"})
main:    -> 0.0010s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20240423020601 RemoveIdxMergeRequestsOnTargetProjectIdAndIidOpened: migrated (0.0409s)

main: == [advisory_lock_connection] object_id: 124200, pg_backend_pid: 38214
ci: == [advisory_lock_connection] object_id: 124660, pg_backend_pid: 38216
ci: == 20240423020601 RemoveIdxMergeRequestsOnTargetProjectIdAndIidOpened: migrating
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0011s
ci: -- indexes(:merge_requests)
ci:    -> 0.0100s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0001s
ci: -- remove_index(:merge_requests, {:algorithm=>:concurrently, :name=>"idx_merge_requests_on_target_project_id_and_iid_opened"})
ci:    -> 0.0021s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0003s
ci: == 20240423020601 RemoveIdxMergeRequestsOnTargetProjectIdAndIidOpened: migrated (0.0264s)

DOWN

(455498-idx_merge_requests_on_target_project_id_and_iid_opened-synchronous-database-index-removal ✗)  ➜  rake db:migrate:down:main VERSION=20240423020601                                 [14:47 23/04]
main: == [advisory_lock_connection] object_id: 123820, pg_backend_pid: 49035
main: == 20240423020601 RemoveIdxMergeRequestsOnTargetProjectIdAndIidOpened: reverting
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0101s
main: -- index_exists?(:merge_requests, [:target_project_id, :iid], {:where=>"state_id = 1", :name=>"idx_merge_requests_on_target_project_id_and_iid_opened", :algorithm=>:concurrently})
main:    -> 0.0083s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- add_index(:merge_requests, [:target_project_id, :iid], {:where=>"state_id = 1", :name=>"idx_merge_requests_on_target_project_id_and_iid_opened", :algorithm=>:concurrently})
main:    -> 0.0020s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20240423020601 RemoveIdxMergeRequestsOnTargetProjectIdAndIidOpened: reverted (0.0310s)

main: == [advisory_lock_connection] object_id: 123820, pg_backend_pid: 49035

(455498-idx_merge_requests_on_target_project_id_and_iid_opened-synchronous-database-index-removal ✗)  ➜  rake db:migrate:down:ci VERSION=20240423020601                                   [14:47 23/04]
ci: == [advisory_lock_connection] object_id: 158000, pg_backend_pid: 49678
ci: == 20240423020601 RemoveIdxMergeRequestsOnTargetProjectIdAndIidOpened: reverting
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0123s
ci: -- index_exists?(:merge_requests, [:target_project_id, :iid], {:where=>"state_id = 1", :name=>"idx_merge_requests_on_target_project_id_and_iid_opened", :algorithm=>:concurrently})
ci:    -> 0.0092s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0001s
ci: -- add_index(:merge_requests, [:target_project_id, :iid], {:where=>"state_id = 1", :name=>"idx_merge_requests_on_target_project_id_and_iid_opened", :algorithm=>:concurrently})
ci:    -> 0.0026s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0002s
ci: == 20240423020601 RemoveIdxMergeRequestsOnTargetProjectIdAndIidOpened: reverted (0.0421s)

ci: == [advisory_lock_connection] object_id: 158000, pg_backend_pid: 49678

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Related to #455498