File head.jsx is a deprecated pattern

using the file [`head.jsx` to define meta tags](https://github.com/pmndrs/react-three-next/blob/main/app/head.jsx) is not valid anymore. The example should be updated to use  the new `metadata` field as explained in the [Next.js docs](https://nextjs.org/docs/app/api-reference/functions/generate-metadata#metadata-fields)