docs: remove npm@6 commands

[sapphi-red]

Description

npm@6 is the default package manager for Node.js 14.x.
Node.js 16.x uses npm@8 and Node.js 18.x/20.x uses npm@9.
Given that Node.js 14.x has reached EOL, I think we can remove this part.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Pull Request Guidelines and follow the PR Title Convention.
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• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
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