Policy Store: Add group-level experiment toggle UI

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Summary

Add a frontend UI toggle for group owners to enable/disable the policy store experiment at the group level.

Context

From !255586 (comment 3883363423):

The backend setting exists (namespace_settings.policy_store_experiment_enabled), but there's no frontend UI for group owners to toggle it yet. Organizations already have this toggle in their settings (ee/app/assets/javascripts/organizations/settings/general/components/policy_store_settings.vue), so we need to add the equivalent for groups.

Technical Details

• Backend: namespace_settings.policy_store_experiment_enabled already exists and works
• Organization UI reference: ee/app/assets/javascripts/organizations/settings/general/components/policy_store_settings.vue
• The toggle should be in the group's security settings (similar location to other security experiment toggles)

Acceptance Criteria

• Add a toggle in group security settings for policy_store_experiment_enabled
• Only show the toggle when the instance-level admin setting is enabled
• Match the UX of the organization-level toggle
• Add appropriate tests