ref(nextjs): Clean up browser tracing integration

[mydea]

Updates the app & pages router instrumentation to avoid depreacted options & streamline it overall.

This required a change to browser tracing, which is to allow to provide a custom sentryTrace / baggage string value to continue a trace from. Before this, we always picked up the trace from meta tag, which meant that if we tried to continue a trace manually in nextjs, then called the default browser tracing integration, it would just overwrite it.

Now, you can optionally pass this like this:

startBrowserTracingPageLoadSpan(
    client,
    { name },
    { sentryTrace, baggage }
});

And this will take presedence over looking at the meta tags! This also means we don't need any custom scope/trace handling in nextjs there anymore, which is also nice!