fix(models): use client/orm instead of adapter/orm

- using new orm api, change some related logics
- newer orm api has the concept of TxOrmer, for purpose of transaction
handling. A transaction ormer in v2 is a stateful object, it should be
dropped after using. A Ormer object is stateless and thread(routine)
    safe, and should not be used for transaction handling. More details
    count be found at official doc:
    https://beego.me/docs/mvc/model/orm.md, and pr note: https://github.com/mindoc-org/mindoc/pull/662#issuecomment-807040262

models/Itemsets.go

@@ -5,7 +5,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/beego/beego/v2/adapter/orm"
+	"github.com/beego/beego/v2/client/orm"
 	"github.com/beego/beego/v2/core/logs"
 	"github.com/mindoc-org/mindoc/conf"
 	"github.com/mindoc-org/mindoc/utils"
@@ -112,8 +112,9 @@ func (item *Itemsets) Delete(itemId int) (err error) {
 	if !item.Exist(itemId) {
 		return errors.New("项目空间不存在")
 	}
-	o := orm.NewOrm()
-	if err := o.Begin(); err != nil {
+	ormer := orm.NewOrm()
+	o, err := ormer.Begin()
+	if err != nil {
 		logs.Error("开启事物失败 ->", err)
 		return err
 	}