Refactor `projects_grade` to unify logic to fetch the `projects`

What does this MR do and why?

We have a bug where archived projects are showing up in the security dashboard. That dashboard should only show stats from un-archived projects.

To fix that bug, I need to exclude archived projects in the query to fetch products in the model.

While looking into that, I noticed the "fetch projects to operate on" logic was spread across the class and instance methods

This refactor uses a lambda to share the logic, so that I can fix the bug in one place in this followup MR

I also added some specs to explicitly document some of the current behavior

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Related to #11478 (closed)