🛠 Repo: Update Netlify Node version from 16 to a supported one

[JoshuaKGoldberg]

Tooling Suggestion Checklist

• [X] I have tried restarting my IDE and the issue persists.
• [X] I have pulled the latest master branch of the repository.
• [X] I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• [X] I have searched for related issues and issues with the faq label, but none matched my issue.
• [X] I want to provide a PR to resolve this

Overview

https://github.com/mochajs/mocha/blob/9fbf3aeb2e6c8c07ced79122f79a1ce73191d0f4/netlify.toml#L17

It'd be great if we could move to the latest even Node version, 20. Let's try for that.

Additional Info

Assigning to myself as an arbitrary Mocha team member.

[voxpelli]

@JoshuaKGoldberg Do we still need this when the new site is ready?

[JoshuaKGoldberg]

👍 we won't

https://github.com/mochajs/mocha/blob/18699a0d668ed2654dd15433f03b74348baf9559/docs-next/netlify.toml#L7