Change minimum node version to 12.10

Webpack memory leak was found with some older Node.js version 12.x. As in #625 (closed), it was fixed using Node.js 12.10 or greater

This PR updates the preflight dependency and the documentation to use Node.js 12.10 or greater.

As a result of this PR, some users might face a preflight-check failure.

Quoting @.luke in #625 (comment 224719379), if you've pinned your homebrew node version as our GDK docs suggested, note that you may get this error:

Error: Not upgrading 1 pinned package:
node 12.10.0

Unpin it first:

brew unpin node@12

Then upgrade node@12:

brew upgrade node@12

And re-pin:

brew pin node@12