Proposal: rename default branches to main

[radu-matei]

As raised by @carolynvs in the July 8th CNAB community meeting, 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.