[WIP] remove to_time_preserves_timezone

[akostadinov]

As a follow up on Rails 7.1 upgrade, lets remove this.

The only place I identified us using #to_time is in

Contract#intersect_with_unpaid_period which is used for variable cost calculations.

This is tested from Finance::VariableCostCalculationTest#variable line items are created.

In that instance the period is actually dates. So #to_time returns the dates in the server local timezone regardless of the to_time_preserves_timezone setting. So the behavior shouldn't change.

But there is still a small chance where #to_time is called somewhere internally. And since providers should be billed in their local timezones,
I think we should simulate that - create a provider with a very different timezone from current and ensure it is billed in that specified timezone.

That's why I leave this a WIP. I'm going away for a week so feel free to pick this up 👼

P.S. there is also Finance::BillVariableForPlanChangedTest where we check whether variable billing would happen in certain timezone conditions. But it doesn't seem to test an actual provider being actually billed on a server with a different time zone.

there is also a test Finance::VariableCostCalculationTest##calculate_variable_cost does the calculations in the timezone of the provider but it is also heavily stubbed so I wouldn't call it trustworthy.

[jlledom]

It looks good to me 👍