change cpm branch: master -> main

[skaji]

I renamed cpm's default branch: master -> main.

[coveralls]

Coverage remained the same at 78.342% when pulling c5ad68a on skaji:cpm into bef20db on gugod:develop.

[gugod]

@skaji Thanks for the heads up.

Would you be so kind and just keep master branch alive for a few more months ? It does not need to be updated at all but since all existing perlbrew version pulls cpm from master branch, install-cpm would be broken if master branch is deleted.

OTOH we could probably setup a redirection URL such is https://cpm.perlbrew.pl to always redirect to a workable version...

[skaji]

I renamed the branch name. See https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch

Then, thanks to github, it seems that the "master" urls will be redirect to the "main" urls,
so we don't necessarily have to change the "master" urls.

Having said that, I think it is a good idea to use canonical urls :)