Upgrade to newest xcode and flutter version

[JulianBissekkou]

Apps using this package are not able to compile if the build is done with xcode 16.

❌  /ios/Classes/SwiftFlutterSystemProxyPlugin.swift:113:26: value of type 'CFRunLoopSource' has no member 'takeUnretainedValue'
            }, &context).takeUnretainedValue()

[oelburk]

Just got the same here!

[JulianBissekkou]

@oelburk
You can test this change by doing:

  flutter_system_proxy:
    git:
      url: https://github.com/tappeddev/flutter_system_proxy.git
      ref: 18-xcode-16-support