[API] Regression - External users no longer show up in /api/v4/users?username=:username

Summary

When not using an admin PAT, external users now (since 13.4) cannot be found anymore when using the /api/v4/users/?username=:username endpoint.

https://docs.gitlab.com/ee/api/users.html#for-normal-users

Steps to reproduce

1. Request user detail via API as normal user from an external user
2. Notice how the external user cannot be found

What is the current bug behavior?

External users no longer show up when looking them up by username

What is the expected correct behavior?

External users should show up when looking them up by username

Output of checks

This bug happens on GitLab.com