Update `index.tsx` with new React 18 docs.

[esteban-gs]

We recently upgraded to React 18 and we will follow up with smaller tickets to solve any parts that may have broken.

In index.tsx, we should render the application root as the React 18 migration docs specify: https://react.dev/blog/2022/03/08/react-18-upgrade-guide#updates-to-client-rendering-apis

I still see it, but I think it's related to a small thing we might have missed when upgrading to React 18. I'll follow up with a ticket.

Originally posted by @esteban-gs in https://github.com/Nonprofit-Exchange-Hub/web-app/pull/351#pullrequestreview-1497519123

Acceptance criteria:

• index.tsx is rendered as specified in docs
• Verify that it works by opening the sign in modal and seeing no errors after closing it.