Refresh security policies only when user is present in approvers

What does this MR do and why?

This MR adds an improvement to Security::RefreshProjectPoliciesWorker by not calling Security::ProcessScanResultPolicyWorker if the project's policies does not have user_approvers in YAML. This also adds a feature flag to skip the worker to mitigate incident: gitlab-com/gl-infra/production#17282 (closed)

MR acceptance checklist

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