Upgrade to MDX 2

[remcohaszing]

Describe the feature you'd like to request

Currently MDX 2 is supported, but everything in Next.js is focused on supporting MDX 1. I would like to update this. I noticed this while implementing #36815.

Describe the solution you'd like

• The with-mdx example is updated to use MDX 2
• The with-mdx-remote example is updated to next-mdx-remote 4
• Preferably @next/mdx uses a dependency on @mdx-js/loader instead of a peer dependency.
• The @next/mdx peer dependency on @mdx-js/react is removed, as it’s redundant now.

I’ll gladly do this myself. Just let me know if you’ll accept a PR for this.

Describe alternatives you've considered

The alternative is not to upgrade, but I think it’s a bad idea to keep suggesting older versions of MDX.