[Backport] [Fix] milestone close timestamp (#8728) (#8731)

* [Fix] milestone close timestamp (#8728)

* BugFix: Update closed_date_unix colum on milestone table on close

* use go standart time lib

* make backport work!

models/issue_milestone.go

 
 import (
 	"fmt"
+	"time"
 
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	}
 
 	m.IsClosed = isClosed
+	if isClosed {
+		m.ClosedDateUnix = util.TimeStamp(time.Now().Unix())
+	}
+
 	if err = updateMilestone(sess, m); err != nil {
 		return err
 	}