chore: update renovate.json

Add support for Git submodules (in beta right now). We don't need to add a token because our submodule is public.

Author: mattwelke

renovate.json

@@ -61,5 +61,8 @@
         "# renovate: datasource=(?<datasource>.+?) depName=(?<depName>.+?)\\s.*?_VERSION ?(\\??=|\\: ?) ?\\\"?(?<currentValue>.+?)?\\\"?\\s"
       ]
     }
-  ]
-}
+  ],
+  "git-submodules": {
+    "enabled": true
+  }
+}