Switch default branch to use when none is supplied to 'main`

[SeanTAllen]

We are in the process of switching ponylang repos over from master
to main as the default branch to match GitHub's change for the
default name for new repos.

This commit switches corral to be in line with the new GitHub defaults.

[ponylang-main]

Hi @SeanTAllen,

The changelog - changed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 171.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.