The AVR core was removed from the main arduino/Arduino GitHub repository, and moved to a separate repository arduino/ArduinoCore-avr. However, the DFU Programming 8U2 page still points to the arduino/Arduino repo as the place to get the ATmega16U2/8U2 firmwares for the UNO and MEGA boards:
Download updated firmware
The latest version is here
Choose the version that is suitable for your board, either arduino-usbserial/Arduino-usbserial-uno.hex or arduino-usbserial/Arduino-usbserial-mega.hex
The file names should probably be changed as well, since all modern Arduinos use the ATmega16U2, not the ATmega8U2.