Update mergify and backport tool for 8.10 (#2523)

Author: lcawl

.backportrc.json

@@ -1,5 +1,5 @@
 {
   "upstream": "elastic/stack-docs",
-  "branches": [{ "name": "8.9", "checked": true }, "8.8", "8.7", "8.6", "8.5", "8.4", "8.3", "8.2", "8.1", "8.0", "7.17", "7.16", "7.15", "7.14", "7.13", "7.12", "7.11", "7.10", "7.9", "7.8", "7.7", "7.6", "7.5", "7.4", "7.3", "7.2", "7.1", "7.0", "6.8", "6.7", "6.6", "6.5", "6.4", "6.3", "6.2", "6.1", "6.0", "5.6"],
+  "branches": [{ "name": "8.10", "checked": true }, "8.9", "8.8", "8.7", "8.6", "8.5", "8.4", "8.3", "8.2", "8.1", "8.0", "7.17", "7.16", "7.15", "7.14", "7.13", "7.12", "7.11", "7.10", "7.9", "7.8", "7.7", "7.6", "7.5", "7.4", "7.3", "7.2", "7.1", "7.0", "6.8", "6.7", "6.6", "6.5", "6.4", "6.3", "6.2", "6.1", "6.0", "5.6"],
   "labels": ["backport"]
 }

.mergify.yml

@@ -15,6 +15,20 @@ pull_request_rules:
             git merge <remote-repo>/{{base}}
             git push <remote-repo> {{head}}
             ```
+  - name: backport patches to 8.10 branch
+    conditions:
+      - merged
+      - base=main
+      - label=backport-8.10
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "8.9"
+        title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
+        labels:
+          - backport
   - name: backport patches to 8.9 branch
     conditions:
       - merged