[chore] Support Node 18

[tomjeatt]

UI won't build with Node 17 or 18 due to a Node security feature (error:0308010C:digital envelope routines::unsupported). As Node 16 has been deprecated and will no longer be supported by Vercel from next year, we will need to change this.

The fix for the security issue is unsupported with Webpack 4 and that means we will either need to upgrade CRA to support Webpack 5 or migrate the build to Vite or similar. Previous attempts to upgrade CRA/Webpack have failed, so it's likely that migrating away from CRA is going to the best solution, although the exact requirements will be confirmed following investigation.

Note: this is blocked by #1522. The Polkadot upgrade should be supported using current build tools, and the build tools upgraded after that has been released.