Proposal: rename default branches to `main`

As raised by @carolynvs in the [July 8th CNAB community meeting](https://github.com/cnabio/community/blob/ca0be21142a4eef26523861dddce5ead6e5e385a/meeting-notes/2020-07-08.md), we propose renaming the default branch name for all repositories in this organization from `master` to `main`.

From the meeting notes:

> - will require CI fixes/updates.
> - will require GH branch protection rules update.
> - after the next successful PR, delete `master`.
> - note: be wary of source links that reference master in the URL.

Start with the referenced meeting notes at 2020-07-08.md, then inventory the organization repositories and their CI configurations, branch protection rules, and source links that reference master. Done means each repository uses main as its default branch, CI and protection settings are updated, and remaining master references are addressed before deletion.

Written by the indexing model from the issue text.