Match Node.js version for Cloudflare Pages and DDEV

We have Node.js v20 here https://github.com/ddev/ddev.com/blob/main/.nvmrc

We should match it to what we have in Cloudflare Pages https://developers.cloudflare.com/pages/configuration/build-image/#supported-languages-and-tools

I recently saw that we are using Cloudflare Pages v1 and it will be upgraded to v3 at some point.