Enable gdk.auto_rebase_projects by default

What does this merge request do and why?

To quote the announcement as part of this MR:

When you run a gdk update, each project it manages performs a git checkout of the default branch and then pulls in the latest changes. This is necessary but isn't a great user experience as any WIP feature branch is switched to the default branch.

The GDK now automatically rebases projects by default, meaning your WIP feature branch won't be switched to the default branch :)

You can disable this functionality by running:

gdk config set gdk.auto_rebase_projects false

For more details, see #2128 (closed).

Closes: Make gdk.auto_rebase_projects enabled by default (#2128 - closed)

How to set up and validate locally

1. Check out this branch ashmckenzie/enable-auto-rebase-by-default
2. Run gdk config set gdk.auto_rebase_projects true
3. In a dependent project, e.g. gitlab/ create a new branch by running git checkout -b test-branch
4. Perform a make update
5. Run a git status under gitlab/ to ensure the current branch is still test-branch

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

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 an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.