docs: Bump `actions/setup-node` from v3 to v4

[anden-akkio]

Bumps the version for the actions/setup-node GitHub Action referenced on the Continuous Integration page from v3 to v4. Many people simply copy-paste the code blocks from this documentation page, so the main benefit is just to have those people use Node's LTS version behind the scenes.

v4 has been around since October of last year, so about half a year ago. You can see the release notes here: https://github.com/actions/setup-node/releases/tag/v4.0.0

Only real change was bumping the Node version and the version of a few dependencies. The interface for providing cache: 'pnpm' does not change, so bumping the version itself should be the only required change as far as pnpm documentation is concerned.

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