Rename master -> main

Rationale: we left dominance behind us.

• Rename master -> main
• Each team member updates their local repository:

# Switch to `master`
$ git checkout master

# Rename it
$ git branch -m master main

# Fetch remote (including new `main` branch):
$ git fetch

# Stop tracking `origin/master`
$ git branch --unset-upstream

# Start tracking `origin/main` instead
$ git branch -u origin/main