[chore] Don't mark PRs with "never stale" label as stale (#34156)

This PR makes the `never stale` label respected on PRs. I noticed on
#32486
that I had added the `never stale` label, but `Stale` was still added.
This is because we need to set the [`exempt-pr-labels`
option](https://github.com/actions/stale?tab=readme-ov-file#exempt-pr-labels)
in the [stale action](https://github.com/actions/stale) workflow.

.github/workflows/close-stale.yaml

@@ -26,6 +26,7 @@ jobs:
           exempt-issue-labels: 'never stale'
           ascending: true
           operations-per-run: 6000
+          exempt-pr-labels: 'never stale'
       - name: Check rate_limit after
         run: gh api /rate_limit