Add actor to enforce_auth_checks_on_uploads FF

What does this MR do and why?

When I originally added this feature flag (!80117 (merged)) we hadn't yet decided that we wanted an incremental percentage-based rollout, which requires an actor. This MR adds it.

This FF was targeted to be used in both Projects::UploadController and Groups::UploadController, but since we'll be incrementing the rollout based on projects, I'm removing the experiment from Groups::UploadController. Since the experiment is to gauge user response to the protected change, this will create an acceptable inconsistency.

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Related to #352291 (closed)