Rename /_next/webpack-hmr to /_next/hmr (#91415)
## What
Rename the HMR WebSocket path from `/_next/webpack-hmr` to `/_next/hmr`.
## Why
The `webpack-hmr` name is a leftover from when webpack was the only
bundler. Now that Turbopack is the default bundler for both `next dev`
and `next build`, the path name is misleading. The generic `/_next/hmr`
better reflects that this endpoint is bundler-agnostic.
## Changes
- **Client source** (`page-bootstrap.ts`, `web-socket.ts`): updated the
WebSocket connection path
- **Server** (`router-server.ts`): updated the HMR request detection
path
- **Turbopack** (`turbopack-dev-server/src/lib.rs`): updated the
fallback WebSocket path check
- **Tests**: updated all test files referencing the old path
- **Docs** (`version-12.mdx`): added a note that the path was renamed to
`/_next/hmr` in Next.js 16, while keeping the original v12 examples
intact
