[Next.js] Emit a warning when the user has not configured onRequestError on Next.js versions that support it

[lforst]

Description

Next.js 15 added onRequestError which allows for deeper instrumentation for errors.

If people upgrade there is nothing telling them to add export const onRequestError = Sentry.captureRequestError; as per our setup guide and wizard: However, it would be very important to do so.

We should add a warning when Next.js builds or in dev mode when onRequestError is missing so that people add it. Only when we are on Next.js >= 15 though

[lforst]

@a-hariti this may be another good one to tackle if you want to familiarize yourself with the SDK a bit!