Changed relevant files for master branch name change to main

[eytanadler]

Purpose

This PR updates github actions and other docs to reflect the change of the default branch name from master to main. The badge names in the readme were changed too, but those may need to be updated with a new link once they're run.

Type of change

What types of change is it?
Select the appropriate type(s) that describe this PR

• Maintenance update

Testing

Explain the steps needed to test the new code to verify that it does indeed address the issue and produce the expected behavior.

Checklist

Put an x in the boxes that apply.

• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation

[codecov]

Codecov Report

Merging #39 (4c12d04) into main (459aa24) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #39   +/-   ##
=======================================
  Coverage   87.86%   87.86%           
=======================================
  Files          47       47           
  Lines        6097     6097           
=======================================
  Hits         5357     5357           
  Misses        740      740           

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