.github: update backport to not run on backport PRs (#988)

Signed-off-by: Rohit Ashiwal <rashiwal@amazon.com>

.github/workflows/backport.yml

@@ -7,7 +7,10 @@ on:
 
 jobs:
   backport:
+    name: Backport
     runs-on: ubuntu-latest
+    # Only react to merged PRs for security reasons.
+    # See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target.
     if: >
       github.event.pull_request.merged
       && (
@@ -20,11 +23,10 @@ jobs:
     permissions:
       contents: write
       pull-requests: write
-    name: Backport
     steps:
       - name: GitHub App token
         id: github_app_token
-        uses: tibdex/github-app-token@v1.5.0
+        uses: tibdex/github-app-token@v2.1.0
         with:
           app_id: ${{ secrets.APP_ID }}
           private_key: ${{ secrets.APP_PRIVATE_KEY }}
@@ -35,5 +37,4 @@ jobs:
         with:
           github_token: ${{ steps.github_app_token.outputs.token }}
           head_template: backport/backport-<%= number %>-to-<%= base %>
-          labels_template: "<%= JSON.stringify([...labels, 'autocut']) %>"
-          failure_labels: "failed backport"
+          failure_labels: backport-failed