Consider changing default branch to `main`

From https://github.com/AlexsLemonade/training-specific-template/pull/132#pullrequestreview-1595818088

> 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!

Start by reviewing the discussion in pull request 132 and inventory every reference to `master` across this repository and the other mentioned repositories. Define the migration scope before making changes; done means the default branch is `main` and affected links are updated.

Written by the indexing model from the issue text.