Add index on projects(id, marked for deletion)

Relates to issue #584085

This migration optimizes queries using the marked_for_deletion_before scope, which was modified in MR !216443 to include projects with pending_delete: true state.

In a future MR, we'll drop the old indexes as part of Drop index `index_projects_aimed_for_deletion` ... (#428732).

Query plans

Before: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/46463/commands/141706

After: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/46463/commands/141734