Ferdi: Adapt app icon for macOS 11.0 Big Sur

[MarceloAvalos]

Is your feature request related to a problem? Please describe.
macOS 11.0 Big Sur is launching over the next few weeks and it introduces a new rounded-rectangle shape for app icons (like iOS app icons, but not 100% equal). Apps that adapt to the new icon specs feel more native and familiar in the OS.

Describe the solution you'd like
Adapt the Ferdi app icon for the macOS build, following the specs provided by Apple.

Describe alternatives you've considered
N/A

Additional context
The app package can only store one icon, and it's fine that users on previous versions of macOS see the new icon (this will happen with most apps as they adapt to the new icon specs.)

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