ci: schedule renovate to run monthly

As I don't have the capacity to check it every week.
And this repository doesn't usually need that much maintenance.

Author: haoqunjiang

Diff for: renovate.json

@@ -1,7 +1,8 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
   "extends": [
-    "github>haoqunjiang/renovate-presets:npm.json5"
+    "github>haoqunjiang/renovate-presets:npm.json5",
+    "schedule:monthly"
   ],
   "ignorePaths": [
     "**/node_modules/**"