Add group-level policy store experiment toggle UI

What does this MR do and why?

Adds a group-level UI toggle for the policy store experiment in the Permissions and group features section of Group Settings.

Problem: Group owners need a way to opt into the policy store experiment without requiring organization-level adoption.

Solution: Adds a checkbox in the group's general settings page (Settings > General > Permissions and group features) to enable/disable the experiment. The checkbox only appears when:

• The group is a root group
• The instance-level admin setting is enabled
• The security_policies_v2 feature flag is on
• The license permits the feature

Screenshots

How to set up and validate locally

1. Enable the feature flag:

   Feature.enable(:security_policies_v2)

2. Enable the admin setting:

   • Go to Admin > Settings > Security and Compliance
   • Expand Security Policies
   • Enable Policy Store experiment

3. Navigate to a root group's General Settings:

   • Go to the group > Settings > General
   • Expand Permissions and group features

4. Verify the Policy store experiment checkbox appears

5. Toggle the checkbox and save:

   • Verify the setting persists after page refresh
   • Only root groups show this setting

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #630303