Consider changing default branch to main

[sjspielman]

From AlexsLemonade/training-specific-template#132 (review)

Semi-related thought: As we are doing all this rejiggering, is it time to update the default branch to main for these repos?

This repo is still using master as its default branch, but it might be good to switch to main at this point. This will involve a non-zero amount of planning, since links to master appear in a couple repos and in a variety of places in those repos. Opening this issue as something we should strongly consider doing, but noting that the full scope of doing so has not been fully explored!