Consolidate virtual registry availability checks with memoization

What does this MR do and why?

Consolidate virtual registry availability checks with memoization

Replace scattered availability logic with centralized methods:

• Add VirtualRegistries.any_registry_available? for unified registry checks
• Add VirtualRegistries::Setting.enabled_for_group? with strong_memoize to avoid repeated DB lookups
• Update show_virtual_registries_setting? helper to delegate to new method
• Update cleanup policy resolver to check both Maven and Container registries
• Update controller and sidebar to use virtual_registry_available? methods
• Moved user_has_access? from individual registry types to parent module

Cleanup [FF] `ui_for_container_virtual_registries` -- A... (#582167 - closed) • Rahul Chanila • 18.11 • On track

EE: true

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #592706 (closed)