ci: add backport pipeline (#1489)

* add backport pipeline

* added automerge
- This will automatically merge backported changes without creating a PR if there are no errors. If there are errors  (merge conflicts, failure to cherrypick, etc.), then a PR will be created requiring manual intervention to resolve

* added a token with elevated permissions

Co-authored-by: Michael Wilkerson <mwilkerson@hashicorp.com>

.github/workflows/backport.yml

@@ -0,0 +1,21 @@
+---
+name: Backport Assistant Runner
+
+on:
+  pull_request:
+    types:
+      - closed
+      - labeled
+
+jobs:
+  backport:
+    if: github.event.pull_request.merged
+    runs-on: ubuntu-latest
+    container: hashicorpdev/backport-assistant:0.2.5
+    steps:
+      - name: Run Backport Assistant
+        run: backport-assistant backport -automerge
+        env:
+          BACKPORT_LABEL_REGEXP: "backport/(?P<target>\\d+\\.\\d+\\.x)"
+          BACKPORT_TARGET_TEMPLATE: "release/{{.target}}"
+          GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}