Fix to .flat() refactor

[medikoo]

• Fix regression introduced with refactor: Replace _.flatten with Array.prototype.flat #11271
• Remove obsolete comment added with refactor: Replace _.flatMap with Array.prototype.flatMap #11272

[codecov]

Codecov Report

Merging #11340 (75cef60) into main (8eae19f) will not change coverage.
The diff coverage is 0.00%.

❗ Current head 75cef60 differs from pull request most recent head 0b8ddf5. Consider uploading reports for the commit 0b8ddf5 to get more accurate results

@@           Coverage Diff           @@
##             main   #11340   +/-   ##
=======================================
  Coverage   85.84%   85.84%           
=======================================
  Files         314      314           
  Lines       13248    13248           
=======================================
  Hits        11373    11373           
  Misses       1875     1875           

Impacted Files	Coverage Δ
lib/plugins/aws/metrics.js	67.24% <0.00%> (ø)
...ts/api-gateway/lib/hack/disassociate-usage-plan.js	100.00% <ø> (ø)

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