No longer compatible with Node < 16 since upgrade to foreground-child > 3.x

All technical details of the test can be found at https://github.com/metalsmith/metalsmith/actions/runs/7854693798/job/21435716699

Since v3.x the dependency `foreground-child` uses the `node:` prefix to include constants and due to this c8 throws in Node < 16:
```
Error: Cannot find module 'node:constants'
```
Shall preferably be tackled after a solution is chosen at https://github.com/tapjs/foreground-child/issues/56