Set container_virtual_registries to default enabled

What does this MR do and why?

In this MR, we set container_virtual_registries as default enabled. In the previous milestone, %18.9, we set the feature flag as beta from wip. In %18.10, we also have rolled out the feature in production.

This feature flag enables the following API endpoints:

• https://docs.gitlab.com/api/container_virtual_registries
• https://docs.gitlab.com/user/packages/virtual_registry/container/#pull-container-images-from-the-virtual-registry

Rollout issue: [FF] `container_virtual_registries` -- Enable t... (#564803)

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Related to #590097 (closed)