Use Namespace#all_projects in Project scope

What does this MR do?

This scope is esentially the same as Namespace#all_projects so we can use that method here.

That method has some optimizations like skipping the CTE if a user namespace is passed in. It also has a workaround for the PG planner bug described in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/12791#note_527286190.

Also see !56078 (merged)

Related to #324220 (closed)

Screenshots (strongly suggested)

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