Update all dependencies to latest version and fix

[charliegerard]

- Summary
This PR updates this repo's dependencies to the latest ones, including Webpack, PostCSS, and Babel.

This way, someone using this template with the latest version of Node (v16 at this point), should get the site running on Netlify properly.

- Test plan

• Clone the repo and run npm install on Node v16 - this step should run successfully.
• Run npm start and the site should open in a new browser window with styling.
• Run the build commands shown in the README and they should run successfully.
• Deploy to Netlify - should deploy successfully and the DP should have the correct styles.
• Run netlify dev locally and it should run the app the same way as npm run start.

- Description for the changelog

Mostly updated versions in the package.json and webpack configs to run with v5.

[ebauger]

Thank you @charliegerard 🙏🏻