At what voltage do the c1 and c2 lines run at when controlling a lego servo motor?
The answer to the question you linked contains the answer you are looking for:


  
  To move in one direction, send a PWM signal (1200 Hz, 0 to 100% duty cycle) on C1 and keep C2 at GND level. As duty cycle varies, servo motor will move along 7 positions on one side. See this video.
  To move in the other direction and reach the 7 other positions, send PWM to C2 and keep C1 at ground level.
  


PWM means Pulse Width Modulation, so you need something capable of generating this signal. The voltage is the same as what you apply to the 9V terminal. Only the pulse width is varied. And GND means "ground" which is another way of saying 0V.

So, to do this with a Raspberry Pi or other microcontroller, you will need a transistor to convert the voltage. There is virtually no load on C1/C2 in this case, so any size of transistor will do the job without risk of overload. See this page for some example circuits.