Revert unrelated changes for SMTP auth (#21767)

The purpose of #18982 is to improve the SMTP mailer, but there were some
unrelated changes made to the SMTP auth in
d60c438694

This PR reverts these unrelated changes, fix #21744

services/auth/source/smtp/source.go

@@ -19,7 +19,7 @@ import (
 // Source holds configuration for the SMTP login source.
 type Source struct {
 	Auth           string
-	Addr           string
+	Host           string
 	Port           int
 	AllowedDomains string `xorm:"TEXT"`
 	ForceSMTPS     bool