Add 8.18 branch to backport and mergify configs (elastic#1664)

Author: karenzone

.backportrc.json

@@ -1,7 +1,7 @@
 {
   "repoOwner": "elastic",
   "repoName": "ingest-docs",
-  "targetBranchChoices": ["main", "8.x","8.17","8.16","8.15","8.14","8.13","8.12", "8.11", "8.10", "8.9", "8.8", "8.7", "8.6", "7.17"],
+  "targetBranchChoices": ["main", "8.x","8.18","8.17","8.16","8.15","8.14","8.13","8.12", "8.11", "8.10", "8.9", "8.8", "8.7", "8.6", "7.17"],
   "autoMerge": true,
   "autoMergeMethod": "squash",
   "branchLabelMapping": {

.mergify.yml

@@ -42,6 +42,20 @@ pull_request_rules:
         title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
         labels:
           - backport
+  - name: backport patches to 8.18 branch
+    conditions:
+      - merged
+      - base=main
+      - label=backport-8.18
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "8.18"
+        title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
+        labels:
+          - backport
   - name: backport patches to 8.17 branch
     conditions:
       - merged