Added missing obsoletions for async methods (#1210)
Added missing obsoletions for async methods (#1210)

Fixes [sc-12578].

## Other changes:
- Cleaned up docs that referenced `completion` on `async` methods
- Fixed incorrect `paymentDiscount(forProductDiscount:)`. The old Swift name was `paymentDiscount(for:)` (Swift automatically removes parameter type from the name)
- Verified all previous (as of version 3.x) `async` methods get a fix-it equivalent to the non-`async` counterparts
RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios