ZHA does support the same standard Zigbee OTA that Philips Hue support, the issue is that you need the official Zigbee OTA firmware image from Philips/Signify for each Philips Hue device that you want to upgrade, and there is the challenge as Philips/Signify as a company still does not publish such official Zigbee OTA firmware images via a public repository or in any other way make them available for third-parties to download.