Sync flatten-array

[angelikatyborska]

The updated test cases only differ by one detail - the test description uses the word "array" instead of "list". I think is is an undesirable change for the Elixir track. I am ignoring those changes, and I have added an appends file to clarify that there are no arrays in Elixir.

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[github-actions]

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[neenjaw]

Agree, the update to array is an unfortunate choice of language. It's too bad a more agnostic word wasn't selected like "collection"