Update macOS icon to match new guidelines #72

Job	Run time
build-macos-brew (clang-9)	3m 14s
build-linux (gcc-9)	7m 48s
build-macos-brew-Qt6 (clang-9)	2m 53s
build-linux-Qt6 (gcc-9)	8m 37s
build-macos-no-brew (clang-9)	6m 44s
build-macos-no-brew-Qt6 (clang-9)	4m 47s
build-windows-mingw (5.15.*)	9m 40s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	10m 55s
build-windows-vs (5.15.*)	8m 44s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	13m 6s
	1h 16m 28s

Provide feedback