We rely on yarn workspaces so we don't want people using npm in the repo by accident.
Also updated the required node version to 12+.
~~Not sure about the min yarn version – the latest one could be missing in various CI-like envs, so I might change it yet.~~
Downgraded yarn to ">= 1.21.1" (the oldest of "current" versions, tagged "legacy")
This is to help with the migration to Ember CLI. In the current running
version of Discourse everything should be the same as before, just with
a few extra files that are not used. However, using Ember CLI this can
be installed as an Ember addon.
Co-Authored-By: Jarek Radosz <jradosz@gmail.com>
