Note that Code Quality images mustn't require auth

What does this MR do and why?

This MR updates the Configure Code Quality to use a private container image registry docs to note that the images must not require authentication.

This clarifies information learned from the #355814 (closed) issue and clarified in two support tickets.

GitLab team members with access to Zendesk can learn more in the tickets:

• https://gitlab.zendesk.com/agent/tickets/300642
• https://gitlab.zendesk.com/agent/tickets/301137

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.