Change default branch name to `main`

[movermeyer]

The Git ecosystem is moving away from the default branch name of master, towards the name main.

e.g. All new repos on GitHub/GitLab/Bitbucket now use main as the default branch name for new repositories.

Change the default branch of iain/http_accept_language to use main.

How is this done?

1. Rename the default branch in the GitHub UI (Requires Administrator rights on the repo)
2. Use git grep master and update mentions of the outdated branch name in documentation and URLs.

Developers with local clones will have to perform a one-time update of the local clones by running:

git branch -m master main
git fetch origin
git branch -u origin/main main
git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/main

These commands are also shown to developers who visit the repo in the GitHub interface, so it doesn't require additional advertising work from our end.