Enable auto-merges for 9.0 rel branches (#57333)

Author: wtgodbe

.github/workflows/inter-branch-merge-flow.yml

@@ -0,0 +1,13 @@
+name: Inter-branch merge workflow
+on:
+  push:
+    branches:
+      - release/**
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  Merge:
+    uses: dotnet/arcade/.github/workflows/inter-branch-merge-base.yml@main

github-merge-flow.jsonc

@@ -0,0 +1,20 @@
+// IMPORTANT: This file is read by the merge flow from main branch only. 
+{
+    "merge-flow-configurations": {
+        // Automate opening PRs to merge release/9.0 to main
+        "release/9.0":{
+            "MergeToBranch": "main",
+            "ExtraSwitches": "-QuietComments"
+        },
+        // Automate opening PRs to merge release/9.0-rc1 to release/9.0
+        "release/9.0-rc1":{
+            "MergeToBranch": "release/9.0",
+            "ExtraSwitches": "-QuietComments"
+        },
+        // Automate opening PRs to merge release/9.0-rc2 to release/9.0
+        "release/9.0-rc2":{
+            "MergeToBranch": "release/9.0",
+            "ExtraSwitches": "-QuietComments"
+        }
+    }
+}