[etherpad] check for new nodejs version compatibility

[noave]

Nodejs 12 / 14 / 16 are EOL and wont be available on Uberspace any longer.

Check if this guide works with Nodejs18 or Nodejs20 and update it accordingly. Otherwise, if you cannot fix it with the new versions mark the guide as broken like it is described here: https://github.com/Uberspace/lab/blob/main/BROKEN_GUIDES.md

(This issue has been created automatically, the guide might already be up to date, in this case you can just comment and close the issue here)

[no-one]

The guide already uses Node.js 18, but I have no problems with Node.js 20 either.

[Uatschitchun]

This can be closed