Add codemod command to middleware deprecation warning (#92532)
## What
Adds the `npx @next/codemod@latest middleware-to-proxy .` command to the
middleware deprecation warning in both `next build` and `next dev`.
## Why
The existing deprecation warning tells developers to use `proxy` instead
of `middleware`, but does not surface the available codemod for
automatic migration. Developers have to click through to the docs page
to discover it. Including the command directly in the warning lets them
copy-paste a one-liner to migrate immediately.
## How
Updated the `Log.warnOnce()` call in both:
- `packages/next/src/build/index.ts` (build path)
- `packages/next/src/server/lib/router-utils/setup-dev-bundler.ts` (dev
server path)
The warning now reads:
```
The "middleware" file convention is deprecated. Please use "proxy" instead.
To migrate automatically, run:
npx @next/codemod@latest middleware-to-proxy .
Learn more: https://nextjs.org/docs/messages/middleware-to-proxy
```
This follows the same pattern used by other Next.js codemod suggestions
(e.g., `next/link` in `link.tsx`).
<!-- NEXT_JS_LLM_PR -->
