Upgrade Storefront to use correct mdx setup

[mimarz]

We are currently using and old way of setting up next with mdx support. This is preventing us from keeping dependencies up to date and we have very old packages here, meaning it will just get worse as next develops.

• https://mdxjs.com/packages/react/

  If you use Next.js, do not use this. Add an mdx-components.tsx (in src/ or /) file instead. See Configuring MDX on nextjs.org for more info.

Use this guide for configuring the new way: https://nextjs.org/docs/pages/building-your-application/configuring/mdx

[Barsnes]

This was done in the next branch