Switch default git branch from master to main

[amandasaurus]

Current default branch on github for this project is master. For the last few years, it's been common to use main as the default. I'm sure you're aware of the reasoning and history, and connentations Github makes it easier to change (cf. guide).

How about switching to main? 😄

[joto]

It's a pain in the neck to change that for everybody who has it checked out. We have enough work to do as it is, we don't need to create extra work.