Fix top issues: Proguard rules, iOS deprecation, and Windows crash

[ponnamkarthik]

This PR addresses three of the top open issues on the repository:

1. Issue [Critical] Toast not visible on production if your app enabled proguard #553: Added documentation to README.md explaining the required Proguard rules for Android release builds (-keep class io.github.ponnamkarthik.toast.** { *; }).
2. Issue UIActivityIndicatorViewStyleWhiteLarge is deprecated: first deprecated in iOS 13.0 #535: Replaced the deprecated UIActivityIndicatorViewStyleWhiteLarge with UIActivityIndicatorViewStyleLarge for iOS 13+ in ios/fluttertoast/Sources/fluttertoast/UIView+Toast.m.
3. Issue using Fluttertoast.cancel on windows will cause Unhandled Exception: MissingPluginException #562: Added a try/catch block for MissingPluginException in fluttertoast.dart to prevent unhandled exceptions when calling cancel() or showToast() on unsupported platforms like Windows.

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PR created automatically by Jules for task 15828502520989948020 started by @ponnamkarthik

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