Consolidate the `aca_ptc` and `premium_tax_credit` variables

[PavelMakarchuk]

They seem to represent the same value - I dont think both variables are necessary and I am unsure which to use

[MaxGhenis]

It's aca_ptc, we're just keeping premium_tax_credit for continuity in case we have API users using the old name. If we have a way to issue a deprecation notice we can apply that, might be an API thing @anth-volk?

[anth-volk]

Unfortunately, we don't have any deprecation feature in the API at the moment. About a year ago, Nikhil and I deprecated something in -uk and just had a printed message - probably not the most sustainable option.

Idea: Can we have a field in a country package (defined on Variable or Parameter via `-core), then consume it upstream?