Fix hasFeature(RetroactiveAttribute) check in iOS 14

[vegaro]

iOS 14 tests are failing with:

❌  /Users/distiller/purchases-ios/Sources/Purchasing/StoreKit1/ProductsFetcherSK1.swift:259:5: unexpected platform condition (expected 'os', 'arch', or 'swift')

#if hasFeature(RetroactiveAttribute)
    ^

see https://app.circleci.com/pipelines/github/RevenueCat/purchases-ios/22627/workflows/1c541690-3b85-4aa2-ad9e-dc7f15ada779/jobs/262246/steps

I think adding a swift compiler check would fix this, but seeing this issues I realized we should maybe change the check to #if $RetroactiveAttribute
swiftlang/swift#69643
https://forums.swift.org/t/hasattribute-only-supports-declaration-attributes/68202

[vegaro]

@RCGitBot please test

[aboedo]

just when I thought I understood Swift

[MarkVillacampa]

Turns out hasFeature was introduced in Swift 5.8, which was first bundled with Xcode 14.3, so it's a pity we need to use Xcode 14.2 in order to run in macOS 12 because iOS 14 sims only run there.

According to SE-0362 which introduced the hasFeature condition, variables with $ are still defined by the compiler to support older tools, however it is not recommended:
https://github.com/swiftlang/swift-evolution/blob/main/proposals/0362-piecemeal-future-features.md#x-instead-of-hasfeaturex

I think a more future-proof fix in this case would be to first check for the swift version, and then the feature. Something like:

#if swift(>=5.8)
#if hasFeature(ReroactiveAttribute)
XXXXXX
#endif
#endif

This is what Apple's swift-nio project is doing

[vegaro]

@RCGitBot please test