Protected containers: Use can_admin_all_resources? instead of user.admin? ContainerRegistryAuthService

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What does this MR do and why?

While working on the feature protected containers, I identified some pontential to improve the code with this MR.

• Apply rubocop recommendations to use can_admin_all_resources? method instead of user.admin?
• Use admin mode in tests admin mode is enabled in tests

🛠 with ❤ at Siemens

MR acceptance checklist

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

MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

Screenshots or screen recordings

Only backend changes.

How to set up and validate locally

1. Execute the test

bundle exec rspec spec/services/auth/container_registry_authentication_service_spec.rb