Fix icon filename

[minorsecond]

Describe the bug
Currently, the icon's name is big_sur_icon.png, reflecting the app's former macOS only status. It should be named buzzbot_icon.png.

Additional context
In res/ directory:

1. Change the name of the icon to buzzbot_icon.png.
2. In buzzbot.desktop, change Icon=big_sur_icon to Icon=buzzbot_icon.