ci: upgrade github checkout action to v4

[michaeltchuang]

Summary

• Upgrade github checkout action to v4 (v3 is deprecated and throwing warnings)

Test Plan

• Check Node.js 16 warning messages go away after next CI codegen run
• Need someone with access to add appropriate label to PR
• Not sure if PR is supposed to target master branch, but that was default

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.88%. Comparing base (f633857) to head (e02e8a9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6010      +/-   ##
==========================================
+ Coverage   55.86%   55.88%   +0.01%     
==========================================
  Files         481      481              
  Lines       68357    68357              
==========================================
+ Hits        38189    38200      +11     
+ Misses      27567    27559       -8     
+ Partials     2601     2598       -3     

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