Update Renovate bot config to update dev dependencies once per month

Doing this to reduce the noise caused by Renovate bot creating several
PRs for packages that are updated frequently.

renovate.json

@@ -6,6 +6,10 @@
     {
       "packageNames": ["automattic/jetpack-autoloader"],
       "rangeStrategy": "bump"
+    },
+    {
+      "depTypeList": ["devDependencies"],
+      "schedule": "every month"
     }
   ]
 }