Use "main" branch instead of "master" branch

[JulienBreux]

Regarding https://sfconservancy.org/news/2020/jun/23/gitbranchname/

[JulienBreux]

Hi @grayside!

Sorry to ping you directly 😬

What do you think about this idea?

[steren]

Considering we have many CI/CD pipelines set up on this repo, if we rename the main branch, we need to update all of these