Use .icon

[kra-mo]

The macOS icon should be updated to use the new .icon format. The asset to use can be found here: nextcloud/ios#4116

@i2h3 I think you would know most about how to do this :)

[kra-mo]

Related: #9127

[i2h3]

@kra-mo Yes. It is trivial to use in a macOS app. I did it often by now. The big headache in regard to Nextcloud is the whole branding and brander part. Customers would need to provide appropriate assets, if they want to leverage that format. And our branding service application is further complicated with additional code switches. 🫣 I need to do some research on what is required in the major and invisible part of this matter. It would probably be good to work on it with #8770 in parallel which also is trivial by itself but not that much anymore given our build tool chain and branding service.