[CI] Update Netlify-build Node.js version

[chalin]

From https://app.netlify.com/projects/prometheus-docs/configuration/deploys#dependency-management:

/cc @juliusv -- I know that there's a new version of the website in the works, but this should probably be updated anyways.

[juliusv]

Wow, that's a Node version number I haven't seen in a looong time :) The old site is completely Ruby-based though and no Node is involved, so I don't think it matters. I just updated it to 22.x in any case.