Remove dependency on semver.herokuapp.com

[edmorley]

The buildpack currently makes a request to semver.herokuapp.com here:

heroku-buildpack-nodejs/lib/failure.sh

Lines 748 to 752 in 34b2aad

	if [ "${npm_version:0:1}" -lt "2" ]; then
	latest_npm="$(curl --silent --get --retry 5 --retry-max-time 15 https://semver.herokuapp.com/npm/stable)"
	warning "This version of npm ($npm_version) has several known issues - consider upgrading to the latest release ($latest_npm)" "https://devcenter.heroku.com/articles/nodejs-support#specifying-an-npm-version"
	mcount 'warnings.npm.old'
	fi

There are a few issues with this:

1. The semver.io site is deprecated and will be sunset in the future (GUS-W-9803092)
2. We should not be relying onherokuapp.com domains in production, since they give reduced flexibility (eg in case of needing to migrate to another app/service etc) and increase the risk of takeover, should the platform app be sunset and the old name not reserved for future use (this step is in the sunset playbook, but still)

It seems the simplest fix would be to make the warning recommend removing the NPM specifier from package.json entirely, thereby relying on the buildpack/Node default - or else to link to the NPM versions page or similar.

cc @joshwlewis @colincasey