docs: pin react version to 18 in Next.js 14 upgrade guide (#74222)
### Improving Documentation
I was just upgrading from Next.js v13 to v14 following this guide:
https://nextjs.org/docs/app/building-your-application/upgrading/version-14
npm installation throws the below error:
```sh
npm error code ERESOLVE
npm error ERESOLVE unable to resolve dependency tree
npm error
npm error While resolving: blog@0.0.8
npm error Found: react@19.0.0
npm error node_modules/react
npm error react@"19.0.0" from the root project
npm error
npm error Could not resolve dependency:
npm error peer react@"^18.2.0" from next@14.2.21
npm error node_modules/next
npm error next@"14.2.21" from the root project
npm error
npm error Fix the upstream dependency conflict, or retry
npm error this command with --force or --legacy-peer-deps
npm error to accept an incorrect (and potentially broken) dependency resolution.
```
The react version for Next.js v14 should be pinned to react@18.