Update Container Registry immutable tags maturity from Beta to GA

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Summary

Update the maturity level of the Container Registry immutable tags feature from Beta to Generally Available (GA) based on successful rollout, internal testing, and early adoption metrics.

Current State

• Feature Status: Live on GitLab.com since GitLab 18.2
• Current Maturity: Beta
• Documentation: https://docs.gitlab.com/user/packages/container_registry/immutable_container_tags
• Feature Flag: container_registry_immutable_tags (enabled by default)

Justification for Maturity Change to GA

Adoption Metrics

• ~50 projects currently using immutable tag rules
• Feature has been successfully rolled out to GitLab.com
• Internal testing completed with positive results

Stability Indicators

• No critical bugs or security issues reported since Beta release
• Feature is functioning as designed across different use cases
• JWT token integration with Container Registry working reliably
• Production-ready stability demonstrated

Customer Feedback

• Multiple customer requests fulfilled (as documented in the original epic)
• Feature addresses key security and compliance requirements
• No significant usability issues reported
• Ready for full production use

Requirements

Documentation Updates

• [ ] Update maturity badge from Beta to GA in documentation
• [ ] Remove any Beta-specific disclaimers or warnings

Acceptance Criteria

• [ ] Documentation reflects GA maturity level
• [ ] Feature is properly categorized as GA in internal systems

Technical Notes

This change is primarily documentation and labeling - no code changes should be required to the core functionality. The feature will continue to operate exactly as it does today.

Related Issues/Epics

• Original implementation epic: &9825
• Documentation: https://docs.gitlab.com/user/packages/container_registry/immutable_container_tags

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