Update backport CI, add PR merged condition (opensearch-project#1952)

Signed-off-by: Shenoy Pratik <[email protected]>
Signed-off-by: Mitchell Gale <[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
@@ -26,5 +27,4 @@ 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