Prefer server-provided authentication for LFS push mirroring

What does this MR do?

LFS servers can set an authenticated: true parameter in their upload and verify instructions to tell the client not to search for credentials to add to the request. However, this is optional - they may simply return instructions containing an Authorization: .... header instead. When this happens, the canonical LFS client doesn't overwrite it with a locally-sourced Authorization header, so we shouldn't either.

This fixes pushing to GitHub, which seems to send the authenticated hint intermittently.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Part of #254281 (closed)