fix legacy React code in src/index.js

[bryceAM]

Dependencies also updated. Everyone will need to npm i again.

[bryceAM]

The old way of rendering using React is currently what's being used...

ReactDOM.render(<App />, document.getElementById('root'));

The new way of rendering using React will be...

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
  <BrowserRouter>
    <App />
  </BrowserRouter>
);