Update pinned node.js version to have access to the `fetch` method

[garyghayrat]

• Update node version #577

• Remove typeof window !== 'undefined' in umbra-js

          Got it. So node.js 18 [now](https://nodejs.org/es/blog/announcements/v18-release-announce) has the fetch API. So an alternative solution is bump the minimum required node version from v16 to v18 and make no other code changes, because the reason we check for existence of the window object is because we need the fetch method: https://github.com/ScopeLift/umbra-protocol/blob/9c0f11e13cb4e0684db6f421d400c3093aacd253/umbra-js/src/classes/Umbra.ts#L383-L385
  

Originally posted by @mds1 in #574 (comment)