Container Registry: Change database.enabled to default to "prefer"

Overview

Update the GitLab Development Kit's Container Registry configuration to default database.enabled to "prefer" as outlined in Epic &19638.

Impacted categories

The following categories relate to this issue:

• 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.

Steps to replicate (optional)

Proposal (optional)

Update GDK's Container Registry configuration to:

• Change the default value of database.enabled from false to "prefer"
• Support the new string values: "true", "false", "prefer"
• Maintain backward compatibility with existing boolean configurations
• Update any GDK documentation or configuration templates

This change enables fresh GDK installations to automatically benefit from the metadata database while ensuring existing development environments continue working without changes.

Environment (optional)

• Operating system name: <!-- output of `uname -a` command -->
• Architecture: <!-- output of `arch` or `uname -m` command -->
• The contents of your gdk.yml (if any)
• Ruby version: <!-- output of `ruby --version` command -->
• GDK version: <!-- output of `git rev-parse --short HEAD` command -->