FF Cleanup - Introduce container registry immutable tags

What does this MR do and why?

The Container Registry: Tag immutability feature (&15139 - closed) is gated behind the container_registry_immutable_tags feature flag. The FF rollout is now complete on gitlab.com, and in this MR, we clean up the FF and release the feature.

References

• Container Registry: Tag immutability feature (&15139 - closed) • Unassigned • 18.1
• [Feature flag] Rollout of `container_registry_i... (#523276 - closed) • João Pereira • 18.2

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How to set up and validate locally

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Related to #523276 (closed)