Update minimum Node version to 20

[wetneb]

For https://github.com/OpenRefine/OpenRefine/pull/6526

[netlify[bot]]

✅ Deploy Preview for openrefine-website ready!

Name	Link
🔨 Latest commit	f31f9918a13f520483662ac319781ecdfb109ea4
🔍 Latest deploy log	https://app.netlify.com/sites/openrefine-website/deploys/661644d64543a800085e9162
😎 Deploy Preview	https://deploy-preview-320--openrefine-website.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[magdmartin]

Since https://github.com/OpenRefine/OpenRefine/pull/6526 was merged three weeks ago, it will be good to keep the documentation up to date. @tfmorris are we good to go with @wetneb proposal?

[tfmorris]

Sure, that's fine.