Update .mergify.yml (#6509)

Per typical protocol, adds `8.18` and points `8.x` backports to `8.19` branch.

Author: jmikell821

.mergify.yml

@@ -25,11 +25,11 @@ pull_request_rules:
           - "backport"
         branches:
           - "main"
-        title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
+        title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})" 
   - name: backport patches to 8.x branch
     conditions:
       - merged
-      - label=v8.18.0
+      - label=v8.19.0
     actions:
       backport:
         assignees:
@@ -39,6 +39,19 @@ pull_request_rules:
         title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
         labels:
           - backport
+  - name: backport patches to 8.18 branch
+    conditions:
+      - merged
+      - label=v8.18.0
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "8.18"
+        title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
+        labels:
+          - backport
   - name: backport patches to 8.17 branch
     conditions:
       - merged