Only run the backport job on merged pull requests

Signed-off-by: Peter Nied <petern@amazon.com>

.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
@@ -25,6 +26,5 @@ jobs:
         uses: VachaShah/backport@v2.2.0
         with:
           github_token: ${{ steps.github_app_token.outputs.token }}
-          branch_name: backport/backport-${{ github.event.number }}
           head_template: backport/backport-<%= number %>-to-<%= base %>
           failure_labels: backport-failed