bump backport-assistant to 0.5.8 (#13471) (#13522)

Co-authored-by: Anurag Sharma <36436571+anurag5sh@users.noreply.github.com>

.github/workflows/backport.yml

@@ -16,7 +16,7 @@ jobs:
       contents: none
     if: github.event.pull_request.merged
     runs-on: ubuntu-latest
-    container: hashicorpdev/backport-assistant:0.5.3
+    container: hashicorpdev/backport-assistant:v0.5.8
     steps:
       - name: Backport changes to stable-website
         run: |