add workflow file for backport-assistant github action (#735)

* add workflow file for backport-assistant github action

* up version of backport-assistant to handle forked PRs

.github/workflows/backport.yml

@@ -0,0 +1,21 @@
+---
+  name: Backport Assistant Runner
+  
+  on:
+    pull_request_target:
+      types:
+        - closed
+  
+  jobs:
+    backport:
+      if: github.event.pull_request.merged
+      runs-on: ubuntu-latest
+      container: hashicorpdev/backport-assistant:0.2.1
+      steps:
+        - name: Run Backport Assistant
+          run: |
+            backport-assistant backport -automerge
+          env:
+            BACKPORT_LABEL_REGEXP: "(?P<target>website)/cherrypick"
+            BACKPORT_TARGET_TEMPLATE: "stable-{{.target}}"
+            GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}