Update svn1 node versions for Gutenberg

As per #4974-meta for future Core builds the NodeJS/NPM versions will need bumping for proper Gutenberg building. The PR won’t be merged prior to the svn build servers being updated.

Gutenberg now requires Node.js 12.x and npm > 6.9.x and the “build server” requires updating to accommodate this.
At the time of creating this ticket:
* the latest Node.js LTS release is 12.14.1 https://nodejs.org/en/
* the latest npm release is 6.13.6 https://blog.npmjs.org/
See also: https://github.com/WordPress/gutenberg/pull/19680

For any questions, you’ll need to follow up with someone more involved with Gutenberg development, see the above Trac ticket or Gutenberg PR.