[in_app_purchase] Fixes StoreKit 2 date format does not match in_app_…

[absar]

…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 flutter/flutter#175072

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• I linked to at least one issue that this PR fixes in the description above.
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• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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