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")
Jarek Radosz