Is there any way to update the Bootcamp assistant app?
Basically Macs what originally were shipped with optical drives should use their drive to install Windows. If your optical drive is not longer available, virtual machine software such as VirtualBox can be substituted, but the installation process becomes significantly more complex. Usually a USB drive can not be used to install any version of Windows on such Macs.

The good news is your machine probably can run Windows 7, 8, 8.1 and 10. If Apple does not support a new versioner of Windows, generally the older drivers can be used to install Windows. I have the current version of Windows 10 running on 2007 and 2011 iMacs.

Also, you probably will need to install Windows to boot using the legacy BIOS boot method. The primary reason for this has to due with the Windows Support Software provided by Apple. The drivers included are designed for a BIOS boot and therefore can cause problems if one attempts as installation using a EFI boot method.