Convert virtual_registry_maven feature flag from from wip to beta

Context

The beta release of the Maven virtual registry MVC (API only interactions) (&14137 - closed) feature is planned in milestone 18.0.

The whole feature is gated behind the virtual_registry_maven feature flag, however, this flag is of WIP type.

We need to convert this flag from being WIP to beta, so that it aligns with the feature release.

Solution

• Move the feature flag YAML file to be under the beta directory, and make sure all constraints are met.
• Check if it's doable to change the actor from user to group.