[examples] Migrate Next.js config.next.js to config.next.mjs

[oliviertassinari]

Steps to reproduce

See https://github.com/mui/material-ui/blob/master/examples/material-ui-nextjs-pages-router-ts/next.config.js

Current behavior

next.config.js file

Expected behavior

next.config.mjs

Context

No response

Your environment

npx @mui/envinfo

  Don't forget to mention which browser you used.
  Output from `npx @mui/envinfo` goes here.

Search keywords: -

[oliviertassinari]

Well, maybe we don't need to do this actually. It's for the Page Router, I did the bulk of the replacements in #40661.

[wojtekfarmery]

Does this still need to be done? I could do it

[oliviertassinari]

This issue is still relevant, a bunch of those are outdated, proof: https://github.com/search?q=org%3Amui+next.config.js&type=code.

[Tomar606]

Hi, I've submitted a pull request to address this issue.
It converts the next.config.js to next.config.mjs.
p.s.- It's my first try contributing in an open source project.