docs: pin react version to 18 in Next.js 14 upgrade guide

[cseas]

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:

npm error code ERESOLVE
npm error ERESOLVE unable to resolve dependency tree
npm error
npm error While resolving: [email protected]
npm error Found: [email protected]
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 [email protected]
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.

[ijjk]

Allow CI Workflow Run

• approve CI run for commit: 05c36fe

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer