no, 21 meters is way too low.
Now, if you land head first, it is more than enough.
The problem is you will not manage to jump head first. It's not easy navigating the jump from that high - it's NOT the same as a jump into the pool. If you try to rotate yourself to fly head first, you will make more than half a rotation, possibly even 2 or 3 rotations, with super hard control. I free falled many times, also I have been to aerodynamic tunnel, with wind 200-300km/h blowing me off from the floor - a long simulation of free fall. Navigating a fall is REALLY hard.
This is why you need to make sure to have 98%+ even when you landed on your back, belly or even legs. And for that, height of 40-50m+ is needed, and don't land on grass, but onto concrete.
Would alcohol help? Sorry, but also not. Alcohol relaxes your muscles, and there are articles saying that people shouldn't be surviving a fall but alcohol made them survive it. Alcohol is good only to overcome SI. This is why you want to go for 40++ meters. To overcome negative effect of alcohol and landing feet first.