Update with-mysql example to Next.js 15, Tailwind 4, Prisma 7 (#88475)
## Summary
Updates the `with-mysql` example to use latest versions of all
dependencies.
### What
- Migrate from Pages Router to App Router
- Update to React 19 and Next.js latest
- Update to Tailwind CSS v4 with `@import "tailwindcss"` syntax
- Update to Prisma 7 with `@prisma/adapter-planetscale` driver adapter
- Convert all files to TypeScript
- Remove unused API route and `vercel.svg`
- Update README to clarify this is for PlanetScale MySQL and link to
[vercel/postgres-next-starter](https://github.com/vercel/postgres-next-starter)
for Postgres users
### Why
The example was significantly outdated:
- Prisma 3.10.0 → Prisma 7 (latest)
- Tailwind 3.0.23 → Tailwind 4
- React 18.2.0 → React 19
- Pages Router → App Router
### How
Followed the official migration guides:
- [Prisma 7 upgrade
guide](https://www.prisma.io/docs/orm/more/upgrade-guides/upgrading-versions/upgrading-to-prisma-7)
- [PlanetScale quickstart with
Prisma](https://www.prisma.io/docs/getting-started/prisma-orm/quickstart/planetscale)
- Aligned configuration files with a fresh `create-next-app` output
## Related
N/A
## How to Test
```bash
cd examples/with-mysql
npm install
# Set up .env with your PlanetScale DATABASE_URL
npx prisma generate
npx prisma db push
npx prisma db seed
npm run dev
```
## Checklist
- [x] Tests and samples are included
- [x] Documentation is updated (README)
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Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>