[BUU] Activate admin_style_v3 for 75% of users

[RachL]

⚠ Please use clockify code #7198 Back Office Uplift (Product) while reviewing or working on this task.

TO BE RELEASED On August 20th
(see schedule)

Description

- As a: super admin
- On page: /admin/feature-toggle/features
- I want to be able to do: See that on top of the admin and new users groups, the toggle is also activated for 75% of users

Acceptance Criteria & Tests

1. Before the PR only admins + new user + 75% + dedicated actors eventually should have had access
2. After: same as 1 + 75% of users instead of 50

We will trust flipper to work on this one, we won't check if it is really 75%