Split migrations folder (#21549)

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

models/migrations/v1_16/v201.go

@@ -0,0 +1,15 @@
+// Copyright 2021 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_16 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func DropTableRemoteVersion(x *xorm.Engine) error {
+	// drop the orphaned table introduced in `v199`, now the update checker also uses AppState, do not need this table
+	_ = x.DropTables("remote_version")
+	return nil
+}