[Fix] milestone close timestamp (#8728) (#8730)

* BugFix: Update closed_date_unix colum on milestone table on close

* go fmt

Author: 6543

models/issue_milestone.go

@@ -306,7 +306,11 @@ func ChangeMilestoneStatus(m *Milestone, isClosed bool) (err error) {
 	}
 
 	m.IsClosed = isClosed
-	if _, err := sess.ID(m.ID).Cols("is_closed").Update(m); err != nil {
+	if isClosed {
+		m.ClosedDateUnix = timeutil.TimeStampNow()
+	}
+
+	if _, err := sess.ID(m.ID).Cols("is_closed", "closed_date_unix").Update(m); err != nil {
 		return err
 	}