Pin sorenlouv/backport-github-action action to ad888e9 (#363)

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

Author: elastic-renovate-prod[bot]

.github/workflows/backport.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Backport Action
-        uses: sorenlouv/[email protected]
+        uses: sorenlouv/backport-github-action@ad888e978060bc1b2798690dd9d03c4036560947 # v9.5.1
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}