Fix bug webhook milestone is not right. (#34419) (#34429)

Backport #34419 by @lunny

Fix #34400

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

routers/web/repo/issue.go

@@ -418,6 +418,16 @@ func UpdateIssueMilestone(ctx *context.Context) {
 			continue
 		}
 		issue.MilestoneID = milestoneID
+		if milestoneID > 0 {
+			var err error
+			issue.Milestone, err = issues_model.GetMilestoneByRepoID(ctx, ctx.Repo.Repository.ID, milestoneID)
+			if err != nil {
+				ctx.ServerError("GetMilestoneByRepoID", err)
+				return
+			}
+		} else {
+			issue.Milestone = nil
+		}
 		if err := issue_service.ChangeMilestoneAssign(ctx, issue, ctx.Doer, oldMilestoneID); err != nil {
 			ctx.ServerError("ChangeMilestoneAssign", err)
 			return