Update FF auto_dismiss_vulnerability_policies to use group actor

Martin Cavojrequested to merge
581377-update-ff-to-use-group-actor into master

What does this MR do and why?

Update FF auto_dismiss_vulnerability_policies to use group actor.

To be able to test the feature on group-level, we should use a group actor consistently for all FF checks.

Frontend uses the group actor already also for project-level policies: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/controllers/projects/security/policies_controller.rb#L19

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