Change default branch name to `main`

It would be good to switch the default branch name here to main, which is a more inclusive term (and is also now the default for new repos on GitHub). This is a continuation from a brief discussion on the co-maintainers issue: https://github.com/dbader/node-datadog-metrics/issues/55#issuecomment-1251689164

[dbader]

✅ Done

[Mr0grog]

I also checked the docs on renaming, and I remembered correctly — it does do redirects. So any links to master will redirect to main: https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch