Use default branch for gitlab-zoekt-indexer

What does this merge request do and why?

Related to gitlab-zoekt-indexer#15 (closed)

We'd like to use the default branch main from gitlab-zoekt-indexer in gdk.

Users can always override this using gdk.yml to lock to a specific version

How to set up and validate locally

1. setup gdk to enable zoekt
2. run gdk reconfigure
3. verify that the main branch is used for building the binary

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.