Rename main branch from master to main

[joeyAghion]

This updates CI and docs to use main instead of master. I used this playbook, plus full-text searching. I'll self-merge to monitor that the main build and staging deploy work as expected, but feedback welcome.

In case anyone encounters hokusai errors that mention the master branch, like:

hokusai staging update --dry-run
ERROR: Not on master branch!  Aborting.

You can pass a --check-branch option:

hokusai staging update --dry-run --check-branch main
Updated Kubernetes environment ... (dry run)

[kajatiger]

Interesting! I never knew about doppler before. So good that you did the name change here. Let's see if this works with hokusai.