Move milestone to models/issues/ (#19278)

* Move milestone to models/issues/

* Fix lint

* Fix test

* Fix lint

* Fix lint

modules/convert/pull.go

@@ -27,7 +27,7 @@ func ToAPIPullRequest(ctx context.Context, pr *models.PullRequest, doer *user_mo
 		err        error
 	)
 
-	if err = pr.Issue.LoadRepo(); err != nil {
+	if err = pr.Issue.LoadRepo(ctx); err != nil {
 		log.Error("pr.Issue.LoadRepo[%d]: %v", pr.ID, err)
 		return nil
 	}