Use Array.flat instead of array-flatten

[almic]

Depends upon #5595, Adopt Node@18 as minimum supported version.

With this new version, we get Array.flat(depth), which can be used instead of a package. Infinity would have the same behavior as the package, recursively unwrapping nested arrays indefinitely. But, perhaps it might be worth selecting a more reasonable value, to avoid a RangeError, such as when an array contains a reference to itself.

Note

Mimimum Node.js version supported is 11

[wesleytodd]

This is going to fail CI for now, but I figured it was better to approve the CI run for reference. I think this is a good change, but we have a bunch on our plate first so please be patient with us as we get the other things done first.