As soon as you plug in your laptop and it starts charging, the voltage will drop down to 19.5V and everything will be okay. There's also a decent chance that 21V is within tolerance for your laptop anyway and it won't be harmed. But, just to remove the risk, you want to make sure it never ever goes higher than 19.5V. Fair enough.
You don't want to drop the input voltage when it's below 19.5V, because then you're needlessly wasting power.
With that in mind, I suggest that you can connect a shunt regulator to the input. A shunt regulator is a device which, when the voltage is too high, draws current to bring the voltage back down. When the voltage is low enough, it doesn't do anything.
The simplest type of shunt regulator is a Zener diode, but remember this has to absorb some of the power from the solar panel when the laptop isn't plugged in, and I don't think they make 50-watt Zener diodes.
The basic design is to put a transistor in parallel with the input, and then adjust the transistor's base current (or gate voltage) until it's dissipating enough power to bring the voltage down to where you want it - something you can do with an op-amp. Putting an extra resistor in series will shift some of the power dissipation to the resistor, which might be more tolerant of high temperatures. Here's a rough draft:
simulate this circuit – Schematic created using CircuitLab
OA1 adjusts the current through R1 and M1 (for some reason I couldn't rename it to Q1) until the + input voltage (1/3 of the supply voltage) matches the Zener voltage (1/3 of 19.5V). Both R1 and M1 will need heatsinks.