Rename master branch to main

[Areustle]

We really should have done this years ago, but now that the pipeline and the tools are in a much more stable place, I think we should rename the master branch in all our fermi-lat repos to main. In addition to the social justice and equity considerations, main is now the default branch name, and renaming is a recommended best practice. The FSSC and SLAC will need to collaborate to manage this for all of the shared repositories.