fix: xorm:version default is inconsistent

The following message is displayed when upgrading to Forgejo:

[W] Table system_setting Column version db default is , struct default is 1

The same message also shows when upgrading from Gitea 1.21 to Gitea 1.22.

It is fine for the version field to default to zero or NULL instead of one.

models/system/setting.go

@@ -18,10 +18,10 @@ import (
 )
 
 type Setting struct {
-	ID           int64              `xorm:"pk autoincr"`
-	SettingKey   string             `xorm:"varchar(255) unique"` // key should be lowercase
-	SettingValue string             `xorm:"text"`
-	Version      int                `xorm:"version"`
+	ID           int64  `xorm:"pk autoincr"`
+	SettingKey   string `xorm:"varchar(255) unique"` // key should be lowercase
+	SettingValue string `xorm:"text"`
+	Version      int
 	Created      timeutil.TimeStamp `xorm:"created"`
 	Updated      timeutil.TimeStamp `xorm:"updated"`
 }