Replace deprecated String.prototype.substr()

[CommanderRoot]

String.prototype.substr() is deprecated so we replace it with String.prototype.slice() which works similarily but isn't deprecated.
.substr() probably isn't going away anytime soon but the change is trivial so it doesn't hurt to do it.

[codecov]

Codecov Report

Merging #1319 (48d6681) into 3.X.X-Branch (0fd618d) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           3.X.X-Branch    #1319   +/-   ##
=============================================
  Coverage         47.83%   47.83%           
=============================================
  Files                34       34           
  Lines              2728     2728           
  Branches            812      812           
=============================================
  Hits               1305     1305           
  Misses             1423     1423           

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[github-actions]

Pull Request Build: 1872543851

• based on latest commit: 48d6681

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[kennethtran93]

Thanks!