Allow overriding the default branch naming conventions

Thanks for stopping by to let us know something could be better!

**Is your feature request related to a problem? Please describe.**

At our organisation our github repositories protect branches with release-*. As the release-please branches are all prefixed release-please these fall under the branch protection rules and the action fails.

**Describe the solution you'd like**

The ability to override the default branch naming conventions, similar to how we can for the commit message.