INCLUSIVITY: Change default branch from `master` to `main`

In support of using more inclusive language, I recommend that efforts be taken across all of `bssw-psip` repositories to change the default branches from `master` to `main`.

No files, tests, or implementation entry points are named. Start by inventorying the bssw-psip repositories and checking each repository's default branch setting. The work is done when the relevant repositories use main and the change is documented or otherwise verified across the organization.

Written by the indexing model from the issue text.