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

Summary

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

Background

The Container Registry will support a "prefer mode" that intelligently selects between database and legacy metadata based on installation context. Omnibus should be updated to use "prefer" as the default value, enabling fresh installations to automatically use the metadata database while preserving existing deployments' behavior.

Implementation

Update the Container Registry configuration in Omnibus to:

• Change the default value of registry['database_enabled'] from false to "prefer"
• Support string values: "true", "false", "prefer"
• Maintain backward compatibility with existing boolean configurations
• Update configuration templates and documentation

Success Criteria

• Omnibus defaults registry['database_enabled'] to "prefer"
• Backward compatibility maintained for existing boolean configurations
• Configuration templates generate proper string values in registry configuration
• Documentation updated to reflect new default and string format options

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