Update React examples to React 18

[AbhiPrasad]

Core or SDK?

Platform/SDK

Which part? Which one?

@sentry/react

Description

@AbhiPrasad since React 18, there's a updated method of importing react.

I took this from the official react website

import { createRoot } from 'react-dom/client';

// Clear the existing HTML content
document.body.innerHTML = '<div id="app"></div>';

// Render your React component instead
const root = createRoot(document.getElementById('app'));
root.render(<h1>Hello, world</h1>);

Originally posted by @Jesse-Box in #6508 (comment)

Suggested Solution

Change everywhere we need to change

[Jesse-Box]

This would be a matter of changing the doc's as the wizard has already made this change.