chore(test-react-app): refactor deprecated call to ReactDOM.render

[nickreynolds]

What issue is this PR fixing

• removes an error that can be safely ignored but can cause confusion

What is being changed

• just refactor how the root of the app is rendered

Quality

Check all that apply:

• [X] I want these changes to be integrated
• [X] I successfully ran pnpm i, pnpm build, pnpm test, pnpm test:browser locally.
• [ ] I allow my PR to be updated by the reviewers (to speed up the review process).
• [ ] I added unit tests.
• [ ] I added integration tests.
• [ ] I did not add automated tests because _________, and I am aware that a PR without tests will likely get rejected.