Drop an index from the deployments table concurrently

What does this MR do and why?

This change is part of an epic to optimise the deployments table: &10185

This will drop the partial_index_deployments_for_legacy_successful_deployments index concurrently. There is no previous MR to drop the index asynchronously in production.

Given that the index has a usage rate of 0 according to Thanos, and its size in production is 16kB, I think this should be okay to drop concurrently.

Index size in production

Screenshots or screen recordings

DB Migration output

main: == [advisory_lock_connection] object_id: 179980, pg_backend_pid: 63243
main: == 20231027013210 RemovePartialIndexDeploymentsForLegacySuccessfulDeployments: migrating 
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.1004s
main: -- indexes(:deployments)
main:    -> 0.0072s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- remove_index(:deployments, {:algorithm=>:concurrently, :name=>"partial_index_deployments_for_legacy_successful_deployments"})
main:    -> 0.0034s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20231027013210 RemovePartialIndexDeploymentsForLegacySuccessfulDeployments: migrated (0.1341s) 

main: == [advisory_lock_connection] object_id: 179980, pg_backend_pid: 63243

How to set up and validate locally

N/A

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