Closes #43091

• Introducing an authentication note to force hiding the username. The AbstractUsernameFormAuthenticator#USERNAME_HIDDEN should be set by authenticators that are executed before the username and password form to make sure their intent is respected when hiding the username field.
• With this note set in the authentication session, the form will always be set LoginFormsProvider.USERNAME_HIDDEN to true.
• We can change places where authenticators are manually setting the LoginFormsProvider.USERNAME_HIDDEN to use this new session note. However, it will require changes not strictly related to the original issue. IMO, we should also enforce a behavior different than what we have today, where once a username is provided, we always show the attempted-username field so that users can restart the flow if they want. I think it is a better UX and I can create an issue for it, if you think it makes sense.
• For now, this note/behavior is very specific to organizations, and when the flow starts with the identity-first login flow.