Upgrade nextjs example to NextJS 14

[smaye81]

This updates the example in nextjs to use Next.js v14. A few minor changes to accommodate v14:

• Accessing dynamic data sources such as cookies is not permitted inside a cache scope, so the getMessages function we use for a fake DB had to be refactored to accept a cookie instead of doing the lookup inside: See https://nextjs.org/docs/app/api-reference/functions/unstable_cache
• serverActions are no longer experimental and are the default so that was removed from the nextjs config file.