Tim Neutkens 89d017eac4 Pages Router: Deprecate React 18 support (#97689) ...

## Summary

- warn during `next dev` and `next build` when React 18 is installed,
while keeping `next start` quiet
- explain that React 18 remains supported in Next.js 16 but will be
removed in Next.js 17
- update the React version guidance with React 19 and TypeScript type
upgrade commands

## Verification

- `NEXT_TEST_PREFER_OFFLINE=1 pnpm test-dev-turbo
test/e2e/react-current-version/react-current-version.test.ts`
- `NEXT_TEST_REACT_VERSION=18.3.1 NEXT_TEST_PREFER_OFFLINE=1 pnpm
test-dev-turbo
test/e2e/react-current-version/react-current-version.test.ts`
- targeted production build/start coverage passed with React 18 and
React 19
- Not passing: `pnpm --filter=next types` (existing unrelated errors in
`httpget.ts`, `image-optimizer.ts`, `proxy-request.ts`, telemetry
typings, and compiled declarations)
- Not passing: `pnpm lint` (existing unused Turbo Tasks check)

<!-- NEXT_JS_LLM -->

2026-08-27 13:00:17 +02:00

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