Revert unrelated changes for SMTP auth (#21767) (#21768)

Backport #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

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

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