BillingConfigItem for v3 still requires future flags to use lineItemBilling

[TravFitz]

This is caused by the code defaulting to the BillingConfigLegacyItem if the flag isn't present, but it isn't an option in v3 and is a requirement.

This is causing my jest testing to go haywire because the types don't match. Is it worth removing the Future for BillingConfigItem in v3?

[lizkenyon]

Hi 👋

Thanks for the flag. I will bring this to the team to review.