circleci: upgrade go version to 1.16

[zchee]

Upgrade go version to 1.16.

[codecov]

Codecov Report

Merging #16 (3a8f03f) into main (81a01c1) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main     #16   +/-   ##
=====================================
  Coverage   54.4%   54.4%           
=====================================
  Files          8       8           
  Lines        498     498           
=====================================
  Hits         271     271           
  Misses       188     188           
  Partials      39      39           

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