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This migrates Flutter to use the `@main` attribute introduced in Swift 5.3. The `@NSApplicationMain` attribute is deprecated and will be removed in Swift 6. See: https://github.com/apple/swift-evolution/blob/main/proposals/0383-deprecate-uiapplicationmain-and-nsapplicationmain.md This change is split into two commits: 1.a508d3e503- This updates the macOS app template and adds a migration to replace `@NSApplicationMain` uses with `@main`. 2.f43482786e- I ran `flutter run -d macos` on each Flutter macOS app in this repository to verify the app migrates and launches successfully. Follow-up to https://github.com/flutter/flutter/pull/146707 Fixes https://github.com/flutter/flutter/issues/143044