Rename master branch to main

[jchristn]

Placeholder to rename the master branch to main. I no longer have repository settings access, so I need assistance from the .NET Foundation maintaining team.

[jchristn]

@dnfadmin @dotnet/dnceng

[mmitche]

@jchristn Can do. Let me know when you want this done.

[jchristn]

Thanks @mmitche when you're ready please pull the trigger

[mmitche]

Done