ci: pin backport action to version instead of default branch

Author: wolfgangwalther

.github/workflows/backport.yaml

@@ -38,7 +38,7 @@ jobs:
 
       # This is required for backport action to cherry-pick the PR
       - name: Fetch PR ref
-        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           ref: ${{ github.event.pull_request.head.sha }}
           token: ${{ steps.app-token.outputs.token }}