Split migrations folder (#21549)

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

models/migrations/v1_18/v226.go

@@ -0,0 +1,15 @@
+// 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 v1_18 // nolint
+
+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
+}