deps: upgrade npm to 6.9.0

Co-authored-by: Myles Borins <mylesborins@google.com>

PR-URL: https://github.com/nodejs/node/pull/26244


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>