Update frontend tooling to use Node 20

[lb-]

Ensure we are on the latest LTS of Node for frontend tooling.

See wagtail/wagtail#11141 (Wagtail core update)

[lb-]

OK this may be blocked by Vercel not supporting Node 20 yet.

https://vercel.com/docs/functions/serverless-functions/runtimes/node-js#node.js-version

https://github.com/vercel/pkg/discussions/1972

[lb-]

Ok. We might be able to use Node 20 now in Vercel.

https://vercel.com/changelog/nodejs-20

[lb-]

OK not sure here, will have to review again later.

Changing Vercel to Node 20 was breaking all the builds so I have just turned it back off for now.

Warning: Detected "engines": { "node": ">=20" } in your `package.json` that will automatically upgrade when a new major Node.js Version is released. Learn More: http://vercel.link/node-version

Error: Command "npm install && python3 -m pip install -r requirements.txt && pip install "urllib3<2" && python3 -m pip install -e ." exited with 1

[laymonage]

Oops, I didn't see that we don't use Node 20 yet. I updated the RTD config to use Node 20 in #284 , builds OK (with some warnings) but I assume we'll want to update it anyway.

[lb-]

I think that's fine, it's just Vercel taking a bit to make their Node 20 support official.

[Stormheg]

Thanks @lb- 👍

I'm doing major spring cleaning on sphinx-wagtail-theme today so I will merge this now.