[in_app_purchase] Fix clang analyzer warnings in iOS platform code #42679
Description
Steps to Reproduce
$ cd packages/in_app_purchase/example/
$ flutter build ios
$ cd ios
$ /usr/bin/xcodebuild build -workspace Runner.xcworkspace -scheme Runner -configuration Release CODE_SIGN_IDENTITY=- CODE_SIGNING_ALLOWED=NO analyzeLogs
InAppPurchasePlugin.m:20:1: warning: Property of mutable type 'NSMutableDictionary' has 'copy' attribute; an immutable object
will be stored instead
@property(copy, nonatomic) NSMutableDictionary *productsCache;
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.