Update icons to new macOS guidelines

[blakegearin]

Summary

• Updated PNG icons to conform to the new macOS guidelines (introduced in Big Sur 11.0)

Links

• Issue: #146
• Documentation: Apple guidelines
• Press release: Design great app icons for macOS Big Sur

Notes

• This was created using the popular Figma community file "macOS Big Sur icon template" by Martin David; I can share if needed

• The gradient from top to bottom is #38BFE3 to #184F69 with a top inner shadow of #1AA2D6; I'm open to other values or generally any other modifications

• The logo is directly pulled from the original 1024px icon, so is a raster instead of a vector; this shouldn't matter since it was never scaled up

[pilotmoon]

Thanks! This looks good.