Update application icon to comply to Big Sur style guide

[codeams]

Example I did:

I'm happy to share a Figma file or .ics if interested in merging.

For curious people who want this icon in their local machine:

1. Download png
2. Find Tomighty in the Applications folder.
3. Right Click, then Get Info
4. Drag and drop PNG file into the Tomato icon at the top left of the Get Info window
5. Restart the App