How to get 20 V from a USB-C charger?
In a word, no. You need to implement the USB Power Delivery protocol through the CC line of the Type-C connector and that's a two way communication at 300kbps complete with preamble, CRC and so on that is pretty much impossible to do without a microcontroller.

The PD message format looks like this:



For full details you'll need the USB Power Delivery specification, but there's a useful introduction here which is where the diagram above came from.