Split migrations folder (#21549)

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

models/migrations/v226.go

@@ -1,15 +0,0 @@
-// Copyright 2022 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 migrations
-
-import (
-	"xorm.io/builder"
-	"xorm.io/xorm"
-)
-
-func fixPackageSemverField(x *xorm.Engine) error {
-	_, err := x.Exec(builder.Update(builder.Eq{"semver_compatible": false}).From("`package`").Where(builder.In("`type`", "conan", "generic")))
-	return err
-}