Add rolling-updates feature flag and compatibility framework

[everettbu]

Test 4

[greptile-apps]

Greptile Summary

This PR introduces a comprehensive rolling updates feature framework for Keycloak with proper feature flag gating. The implementation adds a new ROLLING_UPDATES preview feature flag to the Profile system and implements consistent feature validation across all update-related CLI commands.

The core changes include:

1. Feature Flag Introduction: A new ROLLING_UPDATES feature flag is added to Profile.java with Type.PREVIEW, indicating this is experimental functionality that requires explicit enablement.

2. CLI Command Protection: Both UpdateCompatibilityCheck and UpdateCompatibilityMetadata commands now validate that the rolling updates feature is enabled before execution. When disabled, they provide a standardized error message and exit with code 4.

3. Standardized Error Handling: A new printFeatureDisabled() method is added to AbstractUpdatesCommand to provide consistent error messaging across all update commands. The exit code system is also refined, with RECREATE_UPGRADE_EXIT_CODE changed from 4 to 3, and a new FEATURE_DISABLED exit code of 4 added.

4. Documentation Updates: The update-compatibility guide is updated to warn users that the rolling-updates feature must be enabled, and the new exit code 4 is documented.

5. Infrastructure Changes: The operator test suite is updated to use the new FeatureSpec instead of the deprecated UnsupportedSpec, and the custom Docker image build includes the rolling-updates feature flag.

6. Template Updates: Documentation templates are modified to automatically include the --features=rolling-updates flag in examples.

This implementation follows Keycloak's established patterns for feature flags and provides a solid foundation for rolling updates functionality while maintaining backward compatibility and clear user guidance.

Confidence score: 4/5

• This PR is generally safe to merge with well-structured feature flag implementation and consistent error handling patterns.
• The score is 4/5 due to potential user experience concerns with hardcoded feature flags in documentation templates and the preview nature of the feature requiring careful production consideration.
• Files needing more attention: docs/guides/templates/kc.adoc for the hardcoded feature flag approach, and the operator test changes for potential integration impacts.

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[greptile-apps]

style: Hardcoding --features=rolling-updates in the template removes flexibility. Consider using a parameter or documenting this requirement separately so users understand why this flag is needed.