/api/v1/authn - Change Primary authentication model to validate the posted username against Okta users profile email field instead of login field

Background - most of our Okta users have their login as firstname.lastname and email as a valid email, they access multiple applications, we want to standardized login policy so we decided that login should always be an email, but some of the apps are not yet ready to update the users profile they still want the user to login using their firstname.lastname, so for apps that are ready we want to update their tenant to use the same user profile but instead of updating the user profile (since its shared) just start using the email address to validate against the user profile on a authn call.

What we want to achieve is - current /api/v1/authn validates the username against the login field from the users Okta profile, we want to change this default behavior such that the username we post to the api gets matched with the email field instead of the login field. How can we achieve this. Note - we do not want MFA