Add backport.yml and flow configuration (#2689)

* Add backport.yml and flow configuration

This allows to use the /backport command and to automatically open forward flow PRs from release/10.0 to main

* Add GitHub Actions workflow for backporting PRs

Author: ViktorHofer

.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

github-merge-flow.jsonc

@@ -0,0 +1,10 @@
+// IMPORTANT: This file is read by the merge flow from main branch only. 
+{
+    "merge-flow-configurations": {
+        // Automate opening PRs to merge release/10.0 to main
+        "release/10.0":{
+            "MergeToBranch": "main",
+            "ExtraSwitches": "-QuietComments"
+        }
+    }
+}