fix(auth): Updating ApiKeyCredential to ApiKeyCredentials

[sai-sunder-s]

This change is a part of an effort to Consistently use the term "Credentials" instead of "Credential".

Missed some parts in API Key file in previous PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.29%. Comparing base (fad99a7) to head (29db080).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1706   +/-   ##
=======================================
  Coverage   96.29%   96.29%           
=======================================
  Files          49       49           
  Lines        1888     1888           
=======================================
  Hits         1818     1818           
  Misses         70       70           

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