Merge pull request NixOS#11950 from NixOS/mergify/roberth/config-update

ci(Mergify): configuration update

Author: roberth

.mergify.yml

@@ -90,3 +90,13 @@ pull_request_rules:
           - "2.24-maintenance"
         labels:
           - merge-queue
+
+  - name: backport patches to 2.25
+    conditions:
+      - label=backport 2.25-maintenance
+    actions:
+      backport:
+        branches:
+          - "2.25-maintenance"
+        labels:
+          - merge-queue