[Build] Upgrade Node.js from v10 to v12

[kavilla]

Subtask of: #406

We're currently running Node.js v10.24.1. This sub-issue will track progress towards upgrading to at least v12 on our road to v16. That way if we have a dependency and that requires at least Node.js 12 then we can track it better. Also, the change from v10 to v16 might be large so it breaks it into chunks.

[waynehamadi]

This would be great, there are some functionalities in my code editor I don't get because node is too old.

I have a question : why are we in version 10 ? Kibana upgraded to v12 before the fork :
https://github.com/elastic/kibana/commits/master/.nvmrc
By the way, this is probably what needs to be changed if we bump the version.

[boktorbb]

Closing this issue in favor of a plan to move directly to Node 16 outlined in the parent issue #406