Fix CI issues

[owenniblock]

Motivation

Trying to fix the CI issues we're seeing.

• I fixed the Primer React upgrade issue (<UnderlineNav.Link> becomes <UnderlineNav.Item>).
• I fixed the styled-components upgrade issues

However, this revealed an issue with themes. I suspect this is because the ThemeProvider is coming from @primer/react and build against styled-components@5 so it's not playing nicely with styled-components@6

For now, I've rolled back the styled-components version upgrade. We can try this again when it's updated in the dependencies.

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[keithamus]

Good stuff 👍

[joshfarrant]

Thanks for looking at this @owenniblock 🙌

I've tested removing the optional chaining from live-code.js locally and it appears to work, but if you get chance it would be great to see if the CI is happy with that too