ci: use node version 18.18+

[BobbieGoede]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

After a long search I finally figured out why tests failed using Node version 18.18.2 or higher. Our tests set the Host header to test server responses for different domains, but due to nodejs/undici#2322 this broke as fetch is not allowed to set the Host header for security reasons.

I have added undici as a dev dependency and used it in tests where this behaviour is needed.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[kazupon]

Nice work!