[BUU] Activate admin_style_v3 for dev, test and new servers

[dacook]

When the new Backoffice UI Uplift design (admin_style_v3) is the primary design, we need ensure we are using it while developing new changes:

• dev environments
• tests (local and CI)
• ~~staging~~ (these can be managed manually for now)
• new servers

Steps:

1. Prepare tests
   • [x] https://github.com/openfoodfoundation/openfoodnetwork/pull/11645
   • [ ] spec/system/admin/product_import_spec.rb:100:213:679:720
   • [ ] spec/system/admin/products_spec.rb:283
   • [ ] spec/system/admin/subscriptions/smoke_tests_spec.rb:240
   • Consider improving specs as per review comments

• [ ] Make admin_style_v3 active by default
  • This will enable for dev, test and new servers
  • So we can remove the Flipper.enable in base_spec_helper