refactor: Replace _.flatten with Array.prototype.flat

[pjmattingly]

Addresses: #7747

[codecov]

Codecov Report

Merging #11271 (469c0e0) into main (2ac55b0) will decrease coverage by 0.01%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##             main   #11271      +/-   ##
==========================================
- Coverage   85.89%   85.87%   -0.02%     
==========================================
  Files         315      315              
  Lines       13247    13272      +25     
==========================================
+ Hits        11378    11397      +19     
- Misses       1869     1875       +6     

Impacted Files	Coverage Δ
lib/plugins/aws/metrics.js	67.24% <0.00%> (ø)
lib/plugins/aws/deploy/lib/check-for-changes.js	96.31% <100.00%> (ø)
...package/compile/events/api-gateway/lib/api-keys.js	100.00% <100.00%> (ø)
...ackage/compile/events/websockets/lib/deployment.js	100.00% <100.00%> (ø)
...ib/plugins/aws/utils/get-s3-objects-from-stacks.js	100.00% <100.00%> (ø)
lib/plugins/aws/package/compile/events/http-api.js	96.65% <0.00%> (-0.05%)	⬇️
lib/cli/triage.js	91.34% <0.00%> (ø)
...ns/aws/package/compile/events/cognito-user-pool.js	100.00% <0.00%> (ø)
...urces/resources/cognito-user-pool/lib/user-pool.js	0.00% <0.00%> (ø)
...ib/plugins/aws/custom-resources/resources/utils.js	25.45% <0.00%> (+1.38%)	⬆️

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[pgrzesik]

Thanks @pjmattingly, I have one comment, please let me know what do you think

[pgrzesik]

Thank you @pjmattingly, it looks good 👍