fix(deps): update gaxios dependency

[loganhasson]

gaxios 2.1.0 resolves https://snyk.io/test/github/googleapis/google-auth-library-nodejs

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[loganhasson]

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[codecov]

Codecov Report

Merging #817 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #817   +/-   ##
=======================================
  Coverage   84.54%   84.54%           
=======================================
  Files          17       17           
  Lines         951      951           
  Branches      211      211           
=======================================
  Hits          804      804           
  Misses         87       87           
  Partials       60       60

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