[in_app_purchase] Fixes StoreKit 2 date format does not match in_app_… (#11717)

…purchase_platform_interface

## Description

- As per `PurchaseDetails.transactionDate` api documentation, date should be `Milliseconds since epoch` but the migration to StoreKit2 probably mistakenly changed the date format to "yyyy-MM-dd HH:mm:ss". The issue originated in StoreKit2Translators.swift `extension Transaction convertToPigeon` which was using a magic hardcoded formatter dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
- Fixes both `SK2Transaction.purchaseDate` and `SK2Transaction.expirationDate` and ultimately `PurchaseDetails.transactionDate`

## Tests

Unfortunately it is not testable because the StoreKit 2 Transaction type has no visible initializers, probably thats why there are no tests for StoreKit2Translators.swift `extension Transaction convertToPigeon`

## Related Issues

Fixes https://github.com/flutter/flutter/issues/175072

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