Split migrations folder (#21549)

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

models/migrations/v76.go

@@ -1,75 +0,0 @@
-// 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 migrations
-
-import (
-	"fmt"
-
-	"code.gitea.io/gitea/modules/timeutil"
-
-	"xorm.io/xorm"
-)
-
-func addPullRequestRebaseWithMerge(x *xorm.Engine) error {
-	// RepoUnit describes all units of a repository
-	type RepoUnit struct {
-		ID          int64
-		RepoID      int64                  `xorm:"INDEX(s)"`
-		Type        int                    `xorm:"INDEX(s)"`
-		Config      map[string]interface{} `xorm:"JSON"`
-		CreatedUnix timeutil.TimeStamp     `xorm:"INDEX CREATED"`
-	}
-
-	const (
-		v16UnitTypeCode            = iota + 1 // 1 code
-		v16UnitTypeIssues                     // 2 issues
-		v16UnitTypePRs                        // 3 PRs
-		v16UnitTypeCommits                    // 4 Commits
-		v16UnitTypeReleases                   // 5 Releases
-		v16UnitTypeWiki                       // 6 Wiki
-		v16UnitTypeSettings                   // 7 Settings
-		v16UnitTypeExternalWiki               // 8 ExternalWiki
-		v16UnitTypeExternalTracker            // 9 ExternalTracker
-	)
-
-	sess := x.NewSession()
-	defer sess.Close()
-	if err := sess.Begin(); err != nil {
-		return err
-	}
-
-	// Updating existing issue units
-	units := make([]*RepoUnit, 0, 100)
-	if err := sess.Where("`type` = ?", v16UnitTypePRs).Find(&units); err != nil {
-		return fmt.Errorf("Query repo units: %w", err)
-	}
-	for _, unit := range units {
-		if unit.Config == nil {
-			unit.Config = make(map[string]interface{})
-		}
-		// Allow the new merge style if all other merge styles are allowed
-		allowMergeRebase := true
-
-		if allowMerge, ok := unit.Config["AllowMerge"]; ok {
-			allowMergeRebase = allowMergeRebase && allowMerge.(bool)
-		}
-
-		if allowRebase, ok := unit.Config["AllowRebase"]; ok {
-			allowMergeRebase = allowMergeRebase && allowRebase.(bool)
-		}
-
-		if allowSquash, ok := unit.Config["AllowSquash"]; ok {
-			allowMergeRebase = allowMergeRebase && allowSquash.(bool)
-		}
-
-		if _, ok := unit.Config["AllowRebaseMerge"]; !ok {
-			unit.Config["AllowRebaseMerge"] = allowMergeRebase
-		}
-		if _, err := sess.ID(unit.ID).Cols("config").Update(unit); err != nil {
-			return err
-		}
-	}
-	return sess.Commit()
-}