Allow overriding the default branch naming conventions

[q-leobrack]

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.