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

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[stackblitz]

Run & review this pull request in StackBlitz Codeflow.