Rename default branch from master to main

[jtniehof]

This PR makes changes necessary to rename the default branch from master to main in accordance with current common practice. The actual rename will require me to make a small config change as well.

This is mostly documentation, and a small change to the release support scripts. No changes to dbprocessing functionality are included.

PR Checklist

• [X] Pull request has descriptive title
• [X] Pull request gives overview of changes
• [X] (N/A) New code has inline comments where necessary
• [X] (N/A) Any new modules, functions or classes have docstrings consistent with dbprocessing style
• [X] (N/A) Major new functionality has appropriate Sphinx documentation
• [X] (N/A) Added an entry to release notes if fixing a major bug or providing a major new feature
• [X] (N/A) New features and bug fixes should have unit tests
• [X] (N/A) Relevant issues are linked in the description (use Closes # if this PR closes the issue, or some other reference, such as See # if it is related in some other way)

[jtniehof]

@dnadeau-lanl , if you get a chance to approve/merge this, it's appreciated (and I can do the further changes after), otherwise I can do it in a day or two.

[jtniehof]

Thanks! I've done the rename.