What is a simple voltage limiting circuit?
Instead of dissipating 21W with a series pass element, you could use a buck regulator with the output set to 20V. For input voltages below 20V the regulator would be in dropout and pass the input voltage to the output. for input voltages above 20V it will behave normally as a step down regulator and keep the output at 20V. You need a buck regulator that can operate in drop out which requires that it can operate at 100% duty cycle.