If I open the XCode project, select my application target under “TARGETS”, go to General --> App Icons and click the “Use asset catalog” button there, after the migration, I end up with a Source parameter with the value “Appicon”. When I build now, the app has the correct icon.
I also noticed this in Apple’s docs: “Don’t provide app icons in ICNS or JPEG format.** Add de-interlaced PNG files in the app icon fields of your Xcode project’s asset catalog.” The icon files I selected in the Projucer (small and large) are PNG files in my case (a bit small though: 72x72 and 144x144), so maybe something changed in XCode or the Projucer when it generates the necessary files for the app icon? And then when I click that button in XCode it does its own thing to set up the icons in the right way?
If I now look in the bundle, I see Icon.icns (which was there before I clicked that button) and also Appicon.icns (which was not there before). And the Info.plist now has 2 entries:
Before (with the unmodified Projucer-generated XCode project) I had only this in the Info.plist:
This is of course not a viable solution though.