Always define target gitlab-topology-service/config.toml

What does this merge request do and why?

Previously, make target gitlab-topology-service/config.toml was only defined when gitlab_topology_service.enabled was true.

This makes gdk reconfigure want to change support/makefiles/Makefile.config.mk if it's disabled.

Let's always generate this task.

Follow-up of !3797 (diffs, comment 1987674229).

How to set up and validate locally

1. gdk config set gitlab_topology_service.enabled true
2. gdk reconfigure
3. gdk config set gitlab_topology_service.enabled false
4. gdk reconfigure - no changes ✅

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.