Remove an index on the deployments table asynchronously

What does this MR do and why?

This is part of an epic to optimize the deployments table by removing unused indexes: &10185

This MR is an asynchronous removal migration for index_successful_deployments_on_cluster_id_and_environment_id. This index has a usage rate of 0.

The definition of this index is:

CREATE INDEX index_successful_deployments_on_cluster_id_and_environment_id ON deployments USING btree (cluster_id, environment_id) WHERE (status = 2);

The sync index removal will be done in a follow-up MR.

Screenshots or screen recordings

Migration output

main: == [advisory_lock_connection] object_id: 826200, pg_backend_pid: 46342
main: == 20230818034041 PrepareRemovalIndexSuccessDeploymentsOnClusterIdAndEnvironmentId: migrating 
main: -- index_exists?(:deployments, [:cluster_id, :environment_id], {:name=>"index_successful_deployments_on_cluster_id_and_environment_id"})
main:    -> 0.0078s
main: -- quote_column_name("index_successful_deployments_on_cluster_id_and_environment_id")
main:    -> 0.0000s
main: == 20230818034041 PrepareRemovalIndexSuccessDeploymentsOnClusterIdAndEnvironmentId: migrated (0.0135s) 

main: == [advisory_lock_connection] object_id: 826200, pg_backend_pid: 46342

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #402514 (closed)