Add backport assistant workflow (#235)

Author: prathic-hashicorp

.github/workflows/vault-plugin-backport.yml renamed to .github/workflows/backport.yaml

@@ -3,9 +3,10 @@ on:
   pull_request_target:
     types:
       - closed
-      - labeled
 permissions: write-all
 jobs:
   backport:
     # using `main` as the ref will keep your workflow up-to-date
     uses: hashicorp/vault-workflows-common/.github/workflows/backport.yaml@main
+    secrets:
+      VAULT_ECO_GITHUB_TOKEN: ${{ secrets.VAULT_ECO_GITHUB_TOKEN }}