Drop an index from the deployments table

What does this MR do and why?

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

This will drop the index_successful_deployments_on_cluster_id_and_environment_id index concurrently. It has a usage rate of 0 according to Thanos.

The corresponding async-removal MR is here: !129673 (merged).

I have confirmed that this index is no longer in gprd:

Issue: Remove index_successful_deployments_on_cluster_... (#402514 - closed)

Screenshots or screen recordings

DB Migration output:

main: == [advisory_lock_connection] object_id: 226420, pg_backend_pid: 46281
main: == 20230904103804 DropIndexSuccessfulDeploymentsOnClusterIdAndEnvironmentId: migrating 
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0919s
main: -- indexes(:deployments)
main:    -> 0.0074s
main: -- current_schema()
main:    -> 0.0001s
main: == 20230904103804 DropIndexSuccessfulDeploymentsOnClusterIdAndEnvironmentId: migrated (0.1073s) 

main: == [advisory_lock_connection] object_id: 226420, pg_backend_pid: 46281

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)