Add ability to configure default branch name on gitlab_group resource

Now that gitlab#442298 (closed) has been resolved, please add the ability to configure the default branch name on the gitlab_group resource.

it's somewhat mind boggling that the configuring branch protection defaults was added but not this.

Implementation Plan

• Ensure that go-gitlab exposes the default branch name option for the API
• Update the gitlab_group resource to include the new schema attribute
• Update a gitlab_group test to use the new schema attribute for testing