update yarn, update readme.md on corepack

[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.

[Sharqiewicz]

@pendulum-chain/devs Ready for review. Minor change

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[ebma]

Interesting addition, I wasn't aware of this problem 👍 I tested it locally and encountered some issues with volta but I added a note to the readme to fix this.

[Sharqiewicz]

@ebma Thanks 🦈