Add backport workflow (#791)

Author: MichaelSimons

.github/workflows/backport.yml

@@ -0,0 +1,17 @@
+name: Backport PR to branch
+on:
+  issue_comment:
+    types: [created]
+  schedule:
+    # once a day at 13:00 UTC to cleanup old runs
+    - cron: '0 13 * * *'
+
+permissions:
+  contents: write
+  issues: write
+  pull-requests: write
+  actions: write
+
+jobs:
+  backport:
+    uses: dotnet/arcade/.github/workflows/backport-base.yml@main