Rename default branch master to main

[ken-matsui]

No description provided.

[codecov]

Codecov Report

Merging #376 (00651ee) into main (0eb34be) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #376   +/-   ##
=======================================
  Coverage   78.16%   78.16%           
=======================================
  Files          52       52           
  Lines        2418     2418           
=======================================
  Hits         1890     1890           
  Misses        528      528           

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