Move milestone to models/issues/ (#19278)

* Move milestone to models/issues/

* Fix lint

* Fix test

* Fix lint

* Fix lint

modules/convert/issue_test.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"code.gitea.io/gitea/models"
+	issues_model "code.gitea.io/gitea/models/issues"
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
 	"code.gitea.io/gitea/modules/setting"
@@ -32,7 +33,7 @@ func TestLabel_ToLabel(t *testing.T) {
 }
 
 func TestMilestone_APIFormat(t *testing.T) {
-	milestone := &models.Milestone{
+	milestone := &issues_model.Milestone{
 		ID:              3,
 		RepoID:          4,
 		Name:            "milestoneName",