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

services/proxy/pkg/config/defaults/defaultconfig.go

@@ -50,8 +50,8 @@ func DefaultConfig() *config.Config {
 			Enabled:            true,
 		},
 		AccountBackend:        "cs3",
-		UserOIDCClaim:         "email",
-		UserCS3Claim:          "mail",
+		UserOIDCClaim:         "preferred_username",
+		UserCS3Claim:          "username",
 		AutoprovisionAccounts: false,
 		EnableBasicAuth:       false,
 		InsecureBackends:      false,