Make basic security MR widget available to Starter/Premium users

This issue tracks making the security MR widget available for Starter/Premium users generally, and Free-tier users on .com.

The original implementation in #249543 (closed) made it available for Core users on self-managed instances only.

Implementation plan

• Expose read_vulnerability capability to EE merge request page, via ee/app/serializers/ee/merge_request_widget_entity.rb
• Update EE MrWidgetOptions component to render the basic widget or the EE widget based on that capability
• Verify it works for all tiers:
  • Basic security widget
    • FOSS
    • Free (on EE)
    • Starter
    • Premium
  • EE security widget
    • Ultimate