Align default login attribute across services

Up to now the builtin lico was using the "username" as the login
attribute, while the proxy (and to some extend the auth-basic) service
tried to uniquely identify users by mail address. This aligns the
default configuration of the services to use the username everywhere.

Fixes: #4039
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f