Docs: contributing - setup instructions

[dreamorosi]

What were you searching in the docs?

As reported on Discord (link), the Contributing guide present in the repo recommends users to install npm@8.x in order to contribute to the project.

Since several months however the project has moved to Node.js 18.x, which comes with npm@9.x.

Is this related to an existing documentation section?

https://github.com/awslabs/aws-lambda-powertools-typescript/blob/main/CONTRIBUTING.md#setup

How can we improve?

The project uses Node.js 18 which already comes with a newer `npm` version (9.x), the guide should be updated to reflect this.

Got a suggestion in mind?

No response

Acknowledgment

• I understand the final update might be different from my proposed suggestion, or refused.

[github-actions]

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