Split migrations folder (#21549)

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

models/migrations/v1_13/v142.go

@@ -0,0 +1,25 @@
+// Copyright 2020 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_13 //nolint
+
+import (
+	"code.gitea.io/gitea/modules/log"
+
+	"xorm.io/builder"
+	"xorm.io/xorm"
+)
+
+func SetIsArchivedToFalse(x *xorm.Engine) error {
+	type Repository struct {
+		IsArchived bool `xorm:"INDEX"`
+	}
+	count, err := x.Where(builder.IsNull{"is_archived"}).Cols("is_archived").Update(&Repository{
+		IsArchived: false,
+	})
+	if err == nil {
+		log.Debug("Updated %d repositories with is_archived IS NULL", count)
+	}
+	return err
+}