Deprecate postgresql_clusterwide setting

What does this merge request do and why?

Removes superseded, experiment support/cells-add-secondary script, and related items. Since Cells 1.0 we now have completely independent databases.

Also deprecates the now un-used postgresql_clusterwide setting

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

This change removes the support/cells-add-secondary script. We have already deprecated this in docs.

We will mention this removal in the #gdk channel, not many were using this.

• 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.