fix(api): keep milestone due date when it is omitted in edit request (#39869) (#39921)

Backport https://github.com/go-gitea/gitea/pull/39869

`PATCH /repos/{owner}/{repo}/milestones/{id}` always overwrote the due
date with `due_on`, so a request that only changed other fields, such as
`{"state": "closed"}`, silently removed it.

The due date is now kept when `due_on` is omitted, and `"due_on": null`
or `"due_on": ""` remove it.

Only the milestone fix is backported. The issue and pull request
deadline changes of https://github.com/go-gitea/gitea/pull/39869 build
on https://github.com/go-gitea/gitea/pull/38677, which isn't on v28.

routers/api/v1/repo/milestone.go

@@ -219,7 +219,10 @@ func EditMilestone(ctx *context.APIContext) {
 	if form.Description != nil {
 		milestone.Content = *form.Description
 	}
-	milestone.DeadlineUnix, _ = common.ParseAPIDeadlineToEndOfDay(form.Deadline)
+	if _, err := common.ApplyAPIDeadlineTimestamp(&milestone.DeadlineUnix, form.Deadline, nil); err != nil {
+		ctx.APIErrorAuto(err)
+		return
+	}
 
 	oldIsClosed := milestone.IsClosed
 	if form.State != nil {