Update backport CI, add PR merged condition (opensearch-project#1970)

Signed-off-by: Shenoy Pratik <[email protected]>

.github/workflows/backport.yml

@@ -7,6 +7,7 @@ on:
 
 jobs:
   backport:
+    if: github.event.pull_request.merged == true
     runs-on: ubuntu-latest
     permissions:
       contents: write
@@ -15,15 +16,15 @@ jobs:
     steps:
       - name: GitHub App token
         id: github_app_token
-        uses: tibdex/github-app-token@v1.7.0
+        uses: tibdex/github-app-token@v1.5.0
         with:
           app_id: ${{ secrets.APP_ID }}
           private_key: ${{ secrets.APP_PRIVATE_KEY }}
           installation_id: 22958780
 
       - name: Backport
-        uses: VachaShah/backport@v2.1.0
+        uses: VachaShah/backport@v2.2.0
         with:
           github_token: ${{ steps.github_app_token.outputs.token }}
           head_template: backport/backport-<%= number %>-to-<%= base %>
-          files_to_skip: 'CHANGELOG.md'
+          failure_labels: backport-failed