Introduce 'gdk truncate-legacy-tables' command

What does this merge request do and why?

This MR removes the make truncate-legacy-tables from the gdk reconfigure command as it's not necessary for fresh installations of GDK. Instead, it introduces a new gdk truncate-legacy-tables command to handle truncating unnecessary data in the main and ci databases.

The MR also adds a diagnostic to check for stale data and suggest users to run gdk truncate-legacy-tabls when it's needed.

Closes #2013 (closed)

How to set up and validate locally

1. git checkout 2013-no-db-truncate-on-gdk-bootstrap
2. gdk config set gitlab.rails.databases.ci.enabled true && gdk config set geo.secondary false
3. rm .cache/.truncate_tables
4. gdk doctor

   Stale Data
   ================================================================================
   You might encounter a PG::CheckViolation error during database migrations, likely due to stale data in the ci database that belongs in the main database, or vice versa. To address this, you can run:
   
   gdk truncate-legacy-tables

5. gdk truncate-legacy-tables

   $ gdk truncate-legacy-tables                       
   => Ensuring legacy data in main & ci databases are truncated.
   ...
   ✅️ Legacy table truncation completed successfully.

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.