Update actions/stale action to v9 (#2365)

Co-authored-by: elastic-renovate-prod[bot] <174716857+elastic-renovate-prod[bot]@users.noreply.github.com>
Co-authored-by: Michael Wolf <[email protected]>

Author: elastic-renovate-prod[bot]

.github/workflows/stale.yml

@@ -15,7 +15,7 @@ jobs:
 
     steps:
     - name: "Check PRs"
-      uses: actions/stale@a20b814fb01b71def3bd6f56e7494d667ddf28da # v4
+      uses: actions/stale@28ca1036281a5e5922ead5184a1bbf96e5fc984e # v9
       with:
         repo-token: ${{ secrets.GITHUB_TOKEN }}
         stale-pr-message: |