Verify the "introduction of MultiAttribute login" (alias Attribute login) feature and deprecate the existing "email as a username" feature

Description:
With the introduction of MultiAttribute Login (alias Attribute Login) in the default product (5.12 and 5.10.0 WUM) in the engagement a user gets clear flexibility over controlling username and configure any attribute as his preferred username which is important in PSD2 compliance.

This clears up all the limitations that we had with email as a username feature where email is considered as an immutable attribute.
With the WUM release of the fix, first, we need to verify this and then we should deprecate our existing email as a username feature in our documentation to encourage users to use this attribute Login feature to configure email as their preferred login username.

Suggested Labels:
API-M-4.0.0
API-M-3.2.0

Affected Product Version:
APIM 4.0.0
APIM 3.2.0

Start by locating the documentation for the existing email-as-a-username feature and the MultiAttribute Login or Attribute Login feature for APIM 4.0.0 and 3.2.0. Verify the newer feature and identify the old guidance that should be deprecated. Done means the documentation directs users to configure email through Attribute Login and no longer promotes the deprecated feature.