Prevent re-review and dismiss review actions on closed and merged PRs (#30065)

Resolves #29965.

---
Manually tested this by:
- Following the
[installation](https://docs.gitea.com/next/installation/install-with-docker#basics)
guide (but built a local Docker image instead)
- Creating 2 users, one who is the `Owner` of a newly-created repository
and the other a `Collaborator`
- Had the `Collaborator` create a PR that the `Owner` reviews
- `Collaborator` resolves conversation and `Owner` merges PR

And with this change we see that we can no longer see re-request review
button for the `Owner`:

<img width="1351" alt="Screenshot 2024-03-25 at 12 39 18 AM"
src="bcd9c579-3cf7-474f-a51e-b436fe1a39a4">

(cherry picked from commit 242b331260925e604150346e61329097d5731e77)

routers/web/repo/pull_review.go

@@ -263,6 +263,10 @@ func DismissReview(ctx *context.Context) {
 	form := web.GetForm(ctx).(*forms.DismissReviewForm)
 	comm, err := pull_service.DismissReview(ctx, form.ReviewID, ctx.Repo.Repository.ID, form.Message, ctx.Doer, true, true)
 	if err != nil {
+		if pull_service.IsErrDismissRequestOnClosedPR(err) {
+			ctx.Status(http.StatusForbidden)
+			return
+		}
 		ctx.ServerError("pull_service.DismissReview", err)
 		return
 	}