chore: change default branch from master to main

[peaceiris]

cf. actions/toolkit#514

1. merge this
2. create the main branch and push it git branch -m master main
3. change default branch in the repository settings tab
4. change branch protection rules in the repository settings tab
5. Run https://github.com/ethomson/retarget_prs
6. change the default branch
   • codefactor
   • codecov
   • codeclimate
7. delete the master branch

[codecov]

Codecov Report

Merging #396 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #396   +/-   ##
=======================================
  Coverage   58.09%   58.09%           
=======================================
  Files           4        4           
  Lines         241      241           
  Branches       43       43           
=======================================
  Hits          140      140           
  Misses        101      101           

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