Change default branch to main

[thomas-robinson]

Hello. We have a fork of this repository at the NOAA-GFDL organization, and I would like to change the default branch to main. Would it be possible to update this repo? Instructions can be found here: https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch