Finalize BackfillEventsTargetTypeForProjects batched background migration

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

After the Create `BackfillEventsTargetTypeForProjects` ba... (#565788 - closed) has been completed. We can now finalize the BBM.

Implementation Guide

See https://docs.gitlab.com/development/database/batched_background_migrations/#finalize-a-batched-background-migration on how to finalize the migration.