docs: explain how to specify which npm version Renovate bot uses

[HonkingGoose]

Changes:

• Explain how to tell Renovate which version of npm it should use

Context:

A user wanted to know what version of npm renovate uses for its PRs.
Turns out that there is no way to tell Renovate: "use the default version of npm that comes with the installed node version".

But @rarkins gave a solution: edit your package.json file to have a engines: npm field: #8553 (reply in thread)

I think this solution should be documented.

This can probably be removed once Renovate bot gets a bit smarter with the npm version selection???

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added unit tests, or
• No new tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 24.67.0 🎉

The release is available on:

• GitHub release
• 24.67.0

Your semantic-release bot 📦🚀