Update default branch to 'main'

[dciborow]

trafficstars

Update to align with current repository naming defaults.

[bmoore-msft]

Unfortunately, it's not just a simple rename for this repo - there are thousands of deployments every day that come from the raw github links. This would break the vast majority of those many of which we don't own.. We could have both main and master, but I don't know that the confusion it generates is worth it at the moment.