stale workflow: update permissions (#25287)

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## Description

Fix stale action not closing PRs due to missing `actions: write`
permission

The stale action uses GitHub Actions cache to persist state between
runs. Without `actions: write`, it can't delete its `_state` cache entry
(fails with 403), so the old state accumulates and PRs in that cached
state are permanently skipped with `processed during the previous run`,
meaning they never get evaluated for closing.

Confirmed via Actions logs: PR #23598 was marked stale on April 24 but
has never been evaluated for closing because it appears in the stuck
cache state every run.

## Related issues or tickets

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## Reviews

<!-- Notes for reviewers here -->
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- [ ] Technical review
- [ ] Editorial review
- [ ] Product review

Signed-off-by: Craig Osterhout <craig.osterhout@docker.com>

.github/workflows/stale.yml

@@ -154,6 +154,7 @@ jobs:
     permissions:
       issues: write
       pull-requests: write
+      actions: write  # required for actions/stale to delete its state cache between runs
 
     steps:
       - uses: actions/stale@b5d41d4e1d5dceea10e7104786b73624c18a190f # v10.2.0