Log Info when Issuing Auth Challenge to Clients

Context

As part of normal request flow, the registry will issue 401 errors with instructions for clients to auth, if auth is enabled: https://docs.docker.com/registry/spec/auth/token/

Problem

Currently, the registry will log these at warning level with a message starting with error authorizing context: https://gitlab.com/gitlab-org/container-registry/-/blob/b6c9b386961f2023e01a15eccbe453ac9b438c7c/registry/handlers/app.go#L1000

When an admin is investigating registry issues, these messages can be misleading, interfering with identifying the cause of the problems, delaying resolution.

Proposal

When the error is an auth.Challenge, log a more appropriate message at the info level, such as issuing auth challenge to client: authorization token required instead of the current generic authorization error.