Do foundational work to support gitlab-org/gitlab default branch

What does this Merge Request do and why?

This MR updates the necessary logic to allow gitlab and gitlab-docs to be setup and updated by dynamically determining their default branches.

A few new GDK settings were also added (necessary):

• config.gitlab.auto_update - defaults to true, required by support/component-git-update.
• config.gitlab_docs.enabled - defaults to false, added for consistency.
• config.gitlab_docs.auto_update - defaults to true, required by support/component-git-update.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1141 (closed)