Add mergify backport rule for 2.26

.mergify.yml

@@ -106,3 +106,14 @@ pull_request_rules:
         labels:
           - automatic backport
           - merge-queue
+
+  - name: backport patches to 2.26
+    conditions:
+      - label=backport 2.26-maintenance
+    actions:
+      backport:
+        branches:
+          - "2.26-maintenance"
+        labels:
+          - automatic backport
+          - merge-queue

maintainers/release-process.md

@@ -144,12 +144,10 @@ release:
 
   Make a pull request and auto-merge it.
 
-* Create a milestone for the next release, move all unresolved issues
-  from the previous milestone, and close the previous milestone. Set
-  the date for the next milestone 6 weeks from now.
-
 * Create a backport label.
 
+* Add the new backport label to `.mergify.yml`.
+
 * Post an [announcement on Discourse](https://discourse.nixos.org/c/announcements/8), including the contents of
   `rl-$VERSION.md`.