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

Summary

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

Background

The Container Registry is changing database.enabled from a boolean to a string type that accepts "true", "false", and "prefer" values. The Helm chart 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 the Helm chart to:

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

Success Criteria

• Chart defaults registry.database.enabled to "prefer"
• Backward compatibility maintained for existing boolean configurations
• Chart templates generate proper string values in registry configuration
• Documentation updated to reflect new default and string format options

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