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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