[share_plus] Remove usage of deprecated UIApplication.keyWindow in iOS 13+

[daniel-shuy]

Description

UIApplication.keyWindow is deprecated in iOS 13+, leading to iOS 13+ applications built with the share_plus plugin to emit deprecation warnings on build. In addition, UIApplication.keyWindow is not guaranteed to work in the future.

This PR changes the RootViewController() function to check if the iOS version is >13, and if so, uses the new approach instead of UIApplication.keyWindow.

See https://stackoverflow.com/questions/57134259/how-to-resolve-keywindow-was-deprecated-in-ios-13-0#answer-58031897 for more information.

I have tested this change using the share_plus example on the iOS Simulator.

Related Issues

Fixes #917

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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
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[miquelbeltran]

Thank you! I have merged master into as there was a conflict with the Changelog.

I will release this change under 4.5.0 together with the web share feature

[MujibBasha]

I get the same issue:

`

/Users/builder/programs/flutter/.pub-cache/hosted/pub.dartlang.org/share_plus-6.3.4/ios/Classes/FLTSharePlusPlugin.m:26:46: warning: 'keyWindow' is deprecated: first deprecated in iOS 13.0 - Should not be used for applications that support multiple scenes as it returns a key window across all connected scenes [-Wdeprecated-declarations]
    return [UIApplication sharedApplication].keyWindow.rootViewController;
                                             ^
In module 'UIKit' imported from /Users/builder/clone/ios/Pods/Target Support Files/share_plus/share_plus-prefix.pch:2:
/Applications/Xcode-14.1.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS16.1.sdk/System/Library/Frameworks/UIKit.framework/Headers/UIApplication.h:135:51: note: 'keyWindow' has been explicitly marked deprecated here
@property(nullable, nonatomic,readonly) UIWindow *keyWindow API_DEPRECATED("Should not be used for applications that support multiple scenes as it returns a key window across all connected scenes", ios(2.0, 13.0));
                                                  ^`