Update to yarn 2 (3.x.x)

edit from @kantord:

Use this guide to upgrade our yarn version: https://yarnpkg.com/getting-started/migration

It's a good first issue to anyone who has done such an update before. It does not require any deep knowledge of the project structure. It might be hard for anyone who doesn't know much about yarn and setting up build tools in node

original description from @szabgab :

I tried to set up the application locally. Following https://librelingo.app/docs/ I cloned the repo and then I got the following error:

$ yarn set version classic

Resolving classic to a url...
error An unexpected error occurred: "Release not found: classic".
[yarn-error.log](https://github.com/kantord/LibreLingo/files/7600899/yarn-error.log)
info If you think this is a bug, please open a bug report with the information provided in "/home/gabor/work/LibreLingo/LibreLingo/yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/policies for documentation about this command.

(error log attached)

$ yarn --version
1.22.5
$ node --version
v14.15.1

What do I need to change to make this work?