Update mergify to point to latest backport branch (#13896)

With the release of 1.3.3 and the imminent release of 1.4.0, the regular
backport branch is now `stable/1.4`.

Author: jakelishman

.mergify.yml

@@ -6,4 +6,4 @@ pull_request_rules:
     actions:
       backport:
         branches:
-          - stable/1.3
+          - stable/1.4