Fix approval rules group finder for unauthorized users

What does this MR do and why?

• Contributes to #366741 (closed) (comment 1027072147)
• Sentry error: https://sentry.gitlab.net/gitlab/gitlabcom/issues/3375037

Problem

GroupPolicyPreloader does not correctly handles unauthorized users.

Solution

• Show only public groups to unauthorized users
• Don't preload groups for unauthorized users

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable feature flag Feature.enable(:subgroups_approval_rules)
2. Visit http://localhost:3000/api/v4/projects/6/merge_requests/1/approvals (As an unauthorized user)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.