Switch default branch from 'master' to 'main'

[adamfootdev]

In line with Xcode and GitHub, I've updated the default branch to use 'main' instead of 'master' for git.

[JohnSundell]

@adamfootdev As much as I'd love to change the default branch to main, this particular change that you're proposing is quite problematic, since it'll break all publishing workflows for users who haven't specified a branch manually (which I suspect is the vast majority of users). Instead, I think we should turn that branch argument into an optional, and detect whether to use main or master dynamically, to avoid breaking backward compatibility. Let me know if that's something you'd be interested in updating this PR to do, or if you'd like me to work on it.