Remove container_registry_new_cleanup_service feature flag

📟 Context

In &8379 (closed), we worked towards improving the situation when cleaning up container repositories.

For the whole context, see !98651 (merged).

This change was gated behind a feature flag to have an additional safety net.

The feature flag has been enabled for a week with great success.

Time to remove it from the codebase.

Please note that the feature flag worked on a "switch" (between an old and new service). Given that we have self-managed users that still need the old service, the switch (and both services) will stay in the codebase.

The rollout issue is #375037 (closed)

🔍 What does this MR do and why?

• Remove the container_registry_new_cleanup_service feature flag.
• Update the related spec.

🖥 Screenshots or screen recordings

n / a

⚙ How to set up and validate locally

See !98651 (merged)

🚥 MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.