Switch default branch to `main` (keep `master` as a mirror)

We are going rename our primary/main branch and discontinue using master.

Proposal to move from main (or stable as we're also exploring a beta branch naming convention for community-driven testing). @mcornella can help make a call there.

Given that our install and update process is reliant on branch names, this will not be a quick nor simple change. ...but it will (and needs to) happen. Thanks in advance for your contributions and patience as we figure out a strategy.

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### Tasks
- [ ] Add logic to `upgrade.sh` to locally rename the branch
- [ ] Test branch rename on external fork
- [ ] Test branch rename against zsh plugin managers:
	- [ ] https://github.com/zsh-users/antigen
	- [ ] https://github.com/getantibody/antibody
	- [ ] https://getantidote.github.io/
	- [ ] https://github.com/tarjoilija/zgen
	- [ ] https://github.com/zdharma-continuum/zinit
	- [ ] https://github.com/zplug/zplug
	- [ ] https://github.com/zpm-zsh/zpm
- [ ] Test branch rename against a fork
- [ ] Rename in GitHub UI