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