Add Corepack to readme, update packageManager

[Sharqiewicz]

Problem:

The packageManager field in package.json specifies the package manager and its version for the project. However, it doesn't enforce the use of this package manager on its own.

What to do?

• Update Yarn version
• Add the note below to readme.md:

Enable Corepack by executing the command `corepack enable`. 
Corepack is included by default in Node.js, it manages the Yarn version specified in the `packageManager` field.

**Important:** Modern [Yarn](https://yarnpkg.com/getting-started/install) releases should not be installed globally or via npm - use Corepack instead.