qt6_add_ios_ffmpeg_libraries wrongly uses APPEND on a flag CMake var

inside the qt6_add_ios_ffmpeg_libraries Cmake function, used to deploy FFmpeg dylibs in the iOS bundle, it's using the APPEND variant of set_property() on the XCODE_EMBED_FRAMEWORKS_CODE_SIGN_ON_COPY which is NOT a list but a boolean flag, so it changed its type or something, but in the end, the codesign feature is completely broken on iOS and the libs are NOT signed.

Removing the APPEND and using normal set_property mode works.