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
https://github.com/go-gitea/gitea/pull/18982/commits/d60c43869420f5fc43ad19b454c9ae50dad65964

This PR reverts these unrelated changes, fix #21744

services/auth/source/smtp/source_authenticate.go

@@ -32,7 +32,7 @@ func (source *Source) Authenticate(user *user_model.User, userName, password str
 	var auth smtp.Auth
 	switch source.Auth {
 	case PlainAuthentication:
-		auth = smtp.PlainAuth("", userName, password, source.Addr)
+		auth = smtp.PlainAuth("", userName, password, source.Host)
 	case LoginAuthentication:
 		auth = &loginAuthenticator{userName, password}
 	case CRAMMD5Authentication: