Add more intuitive message for disabled password authentication

Release notes

Add more intuitive header message (than "Invalid username or password") for Enterprise users failing to login to gitlab.com, while disabled password authentication is enabled on their top-level group SAML SSO settings.

Problem to solve

Enterprise users are confused why they are not allowed to login when the mentioned setting is enabled. Seeing the "Invalid username or password" header is misleading as it shows the same for both invalid and valid credentials.

Proposal

Introduce a more intuitive header message for Enterprise users failing to login to gitlab.com

E.g. "Password authentication is disabled for your Enterprise user account. Please go to gitlab.com/<enterprise-group> to login via your IdP (SSO)".

This way they are aware that the mentioned setting has been enabled, and they should login as described.

Intended users

Enterprise users on gitlab.com.

Feature Usage Metrics

Does this feature require an audit event?

N/A