Add support for self-updating to `main`

What does this Merge Request do?

We add a quick check as to whether a main branch exists. If it does indeed exist, we check whether it is ahead of master and then use it.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.