Schedule temporary index for BackfillEventsTargetTypeForProjects for removal

See 'target_type' parameter for Events API is nil f... (&19098) for context

Currently, project events are created without a set target_type and target_id. This is causing a problem with our Events RESP API as the target_type filter always gives an empty result for project.

Once Finalize `BackfillEventsTargetTypeForProjects` ... (#565789 - closed) has been completed. We can now remove this temporary index.

Implementation Guide

1. Create a new migration to schedule the removal of the index added in Create temporary index for `BackfillEventsTarge... (#565790 - closed)

See https://docs.gitlab.com/development/database/adding_database_indexes/#schedule-the-index-to-be-removed