Split migrations folder (#21549)

There are too many files in `models/migrations` folder so that I split
them into sub folders.

models/migrations/v1_6/v72.go

@@ -0,0 +1,31 @@
+// Copyright 2018 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package v1_6 // nolint
+
+import (
+	"fmt"
+
+	"code.gitea.io/gitea/modules/timeutil"
+
+	"xorm.io/xorm"
+)
+
+func AddReview(x *xorm.Engine) error {
+	// Review see models/review.go
+	type Review struct {
+		ID          int64 `xorm:"pk autoincr"`
+		Type        string
+		ReviewerID  int64 `xorm:"index"`
+		IssueID     int64 `xorm:"index"`
+		Content     string
+		CreatedUnix timeutil.TimeStamp `xorm:"INDEX created"`
+		UpdatedUnix timeutil.TimeStamp `xorm:"INDEX updated"`
+	}
+
+	if err := x.Sync2(new(Review)); err != nil {
+		return fmt.Errorf("Sync2: %w", err)
+	}
+	return nil
+}