Update to React version 18

[adamstankiewicz]

Description

Update React version, test manually and fix tests

Model after this PR

Ideally, will be additive-only, ensuring Paragon is backwards-compatible with React 16/17 still such that consuming applications aren't forced to upgrade to React 18 just to upgrade Paragon (pretty much all of Open edX frontend is still on React 16/17).

Merge Checklist

• [ ] If your update includes visual changes, have they been reviewed by a designer? Send them a link to the Netlify deploy preview, if applicable.
• [ ] Does your change adhere to the documented style conventions?
• [ ] Do any prop types have missing descriptions in the Props API tables in the documentation site (check deploy preview)?
• [ ] Were your changes tested using all available themes (see theme switcher in the header of the deploy preview, under the "Settings" icon)?
• [ ] Is there adequate test coverage for your changes?
• [ ] Consider whether this change needs to reviewed/QA'ed for accessibility (a11y). If so, please add wittjeff and adamstankiewicz as reviewers on this PR.

Post-merge Checklist

• [ ] Verify your changes were released to NPM at the expected version.
• [ ] If you'd like, share your contribution in #show-and-tell.
• [ ] 🎉 🙌 Celebrate! Thanks for your contribution.