Allow authenticating with password+OTP for git+HTTP access

What does this MR do?

By default, we use username & password for git+HTTP operations. Once the user sets up 2FA, we require a personal access token instead of a password. If the user provides a password in this case, we return an error message to instruct what needs to be changed.

This allows using OTP for git+HTTP. The authentication schema in git is fixed, so we can't request a new prompt for OTP from GitLab. Instead, we could require users to append the OTP code to the password or personal access token. If the OTP code is missing or incorrect, we could return an error message to avoid confusion.

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