Cannot update project mirroring pull URL for public projects

Summary

The project mirroring update page only supports URLs that include username. Related issues: https://gitlab.com/gitlab-org/gitlab-ee/issues/3504 and https://gitlab.com/gitlab-org/gitlab-ee/issues/2549

Steps to reproduce

• Import a private project, set up mirroring right away from the import page
• On the remote server, make the project public
• In the project mirroring pull settings, remove the username from the mirror url and save

What is the current bug behavior?

The page saves successfully, but if when you check, the username is still present in the URL.

What is the expected correct behavior?

The page should allow you to save any valid git mirror URL

Relevant logs and/or screenshots

Customer has posted a screenshots/gif in this issue: https://gitlab.zendesk.com/agent/tickets/82079