chore(cna): switch to .mjs tailwind config (#72124)
### What?
Tailwind CSS [supports an ESM
config](https://tailwindcss.com/docs/configuration#using-esm-or-type-script)
file.
This PR changes `tailwind.config.js` to `tailwind.config.mjs`.
### Why?
1. To convert one more file to ESM
2. To use the modern format
3. To follow other similar migrations that have taken place in the
Next.js codebase (e.g. `next.config.mjs`)
### How?
1. Change file extensions from `.js` to `.mjs`
2. Change module format from CommonJS to ESM
Co-authored-by: Sam Ko <sam@vercel.com>
