Delete Todos for guest users when project visibility
level is updated to private.

`Todos::Destroy::EntityLeaveService#project_ids` was returning
ActiveRecord objects with IDs, not simply IDs. That means we were
serialising more than we needed to in Sidekiq.

We can simply rename this method to `#projects` as this class doesn't
use any of the superclass methods that would use `#project_ids`.

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

Partially addresses #47424.

- dont remove todos for authos & assignees
- remove todos for project guests