Default branch has been renamed to main

[dougbu]

On Thursday, 21 January 2021 we're going to rename the default branch in this repository to main. For more details, see our earlier announcement. Expect the process to begin after 5PM Pacific.

[dougbu]

Some cleanup remains but the work is mostly done. Will leave this issue open even after absolutely everything is complete to make the change very visible. See also the following on the repo home page