[macOS] Update template icons to Big Sur style (#103739)

Updates the AppIcon assets in the macOS app template to meet Apple's
icon guidelines as of macOS Big Sur. Assets were moved to the
[flutter_template_images][1] package, which includes them as of version
4.1.0.

Ref: https://developer.apple.com/design/human-interface-guidelines/macos/icons-and-images/app-icon/

Issue: https://github.com/flutter/flutter/issues/103371

[1]: https://github.com/flutter/packages/tree/main/packages/flutter_template_images