Upgrade package "node-fetch" to fix

[JgJun]

GHSA-r683-j2x4-v87g suggests to us 2.6.7 or above to fix vulnerable to Exposure of Sensitive Information to an Unauthorized Actor

Ember-fetch is on 2.6.1
https://github.com/ember-cli/ember-fetch/blob/master/package.json#L40

In order to fix the CVE the dependency should be upgraded.