[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
   • [BUU] Activate admin_style_v3 for most system specs #11645
   • spec/system/admin/product_import_spec.rb:100:213:679:720 - Use application tmp dir for product import #12649
   • spec/system/admin/products_spec.rb:283 - [BUU] Activates admin_style_v3 for dev, test and new servers #12652
   • spec/system/admin/subscriptions/smoke_tests_spec.rb:240 - [BUU] Activates admin_style_v3 for dev, test and new servers #12652
   • 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

[dacook]

Once #11645 is complete all tests are updated, we should be able to simply make admin_style_v3 ACTIVE_BY_DEFAULT in feature_toggle.rb

[filipefurtad0]

As per delivery circle, I've assigned myself to this issue @dacook .