Drop index_projects_on_created_at_and_id for projects

What does this MR do and why?

Drops index_projects_on_created_at_and_id for projects table, as part of Reduce LockManager LWLock Contention (&11543 - closed).

index_projects_on_created_at_and_id" btree (created_at, id) is covered by index_projects_api_created_at_id_desc" btree (created_at, id DESC)

Check #428827 (comment 1674300979)

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 #428827 (closed)