configure renovate to update github actions monthly

Author: marcoieni

.github/renovate.json5

@@ -0,0 +1,13 @@
+{
+  $schema: "https://docs.renovatebot.com/renovate-schema.json",
+  extends: [
+    "github>rust-lang/renovate:actions",
+    "docker:disable",
+  ],
+  "packageRules": [
+    {
+      "matchManagers": ["github-actions"],
+      "extends": ["schedule:monthly"]
+    }
+  ],
+}