Remove Node.js 18.x support

[davidjgoss]

🤔 What's changed?

Remove Node.js 18 support by removing the testing.

⚡️ What's your motivation?

Node.js 18 is now end of life per https://nodejs.org/en/about/previous-releases. Dropping it enables us to adopt new platform features and get rid of debt.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)
• 💥 Breaking change (incompatible changes to the API)

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

---

This text was originally generated from a template, then edited by hand. You can modify the template here.