How do I make a flat list out of a list of lists?

I have a list of lists like

[
    [1, 2, 3],
    [4, 5, 6],
    [7],
    [8, 9]
]

How can I flatten it to get [1, 2, 3, 4, 5, 6, 7, 8, 9]?

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_{If your list of lists comes from a nested list comprehension, the problem can be solved more simply/directly by fixing the comprehension; please see How can I get a flat result from a list comprehension instead of a nested list?.}

_{The most popular solutions here generally only flatten one "level" of the nested list. See Flatten an irregular (arbitrarily nested) list of lists for solutions that completely flatten a deeply nested structure (recursively, in general).}