iOS build using macos-12 runner now fails

[softins]

Github have now discontinued support for the macos-12 runner, so we need to move those builds to at least macos-13.

Just raising this issue to give it visibility.

[ann0see]

Time to update the iOS build to Qt6 then.
Not sure about macOS legacy.

[softins]

#3436 fixes the build for MacOS Legacy.

I tried the same fix for iOS, but the build failed for other reasons, which I have not yet understood.

[softins]

Now that the macOS Legacy build has been successfully moved to the macos-13 runner, have updated the issue title to just refer to iOS.

Don't yet know what the right solution is for iOS. Needed also to change the SDK version from 13 to 14, and the build no longer succeeded.

[ann0see]

Updating to Qt6, I suppose.
Otherwise building Qt 5.15 from source.

There are still some open PRs and issues with the Qt6 build.

[softins]

See also #3293 and the comments there about the need to move from the "legacy build system" to the "modern build system" when using SDK 14.

[ann0see]

I've tried to compile Jamulus locally on macOS with the 18.2 SDK and it fails with a linker error related to ffmpeg. (Will try to debug it further and then open an issue) I do not remember that we use ffmpeg anywhere.

The qtmultimedia plugin might pull it in.