Update Node.js to v20

[WillBAnders]

Version upgrade for a major dependency Node.

Requirements

• Review the v19.0.0/v20.0.0 changelogs for any notable breaking changes (very unlikely any of these affect us but is good practice to do when updating dependencies; especially across major versions).
• Update package.json to add an engines section for Node.js (above "scripts"); see this StackOverflow post for details. Note that this is only a warning, but that's okay - this is mainly to document the supported Node.js version and allows cross-checking the version with the one used by GitHub Actions.
• Update the lint/test actions to use node-version: 20.x.