Introduce maven_virtual_registry beta feature flag

Context

The beta version of the Maven virtual registry MVC (API only interactions) (&14137 - closed) feature is ready for release.

The whole feature is currently gated behind the virtual_registry_maven feature flag. However, there are some issues that we need to address:

• It's a WIP feature flag, and we need a beta feature flag.
• Its YAML definition file is located in the ce side of the code base. Since the Maven VReg feature is a premium+, we need the feature flag to be defined in the ee side of the codebase.
• Its actor is the current user. Switching to a Group actor would make the rollout way easier for customers.

In order to fix those issues, we replace the virtual_registry_maven feature flag with a new one: maven_virtual_registry. The new feature flag has the following characteristics:

• It's a beta feature flag.
• It has the group as an actor.
• It lives in the ee side of the codebase.

What does this MR do and why?

• Replace the WIP virtual_registry_maven feature flag with beta maven_virtual_registry.
• Update all occurrences in the codebase and documentation to have the new feature flag name.
• Remove the feature flag check from the application setting rate limit UI.

References

• Maven virtual registry MVC (API only interactions) (&14137 - closed) • David Fernandez, Moaz Khalifa • 18.1
• Convert virtual_registry_maven feature flag fro... (#540276 - closed) • Moaz Khalifa • 18.1

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Try to create a new registry on a top level group:

   curl -v -i \
     -H "PRIVATE-TOKEN:<PAT>" \
     -H "Content-Type: application/json" \
     -d '{"name": "foo"}' \
     "http://gdk.test:3000/api/v4/groups/<group_id>/-/virtual_registries/packages/maven/registries"
   
   # {"message":"401 Unauthorized"}% 

2. Enable the flag:

   Feature.enable(:maven_virtual_registry)

3. Repeat the same curl command, the registry should be created successfully.

MR acceptance checklist

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Related to #540276 (closed)