The last official version of Nvidia flex plugin is for 4.19 version. You can’t just change version of the engine, the plugin must be fixed for the new version. It is possible to find 4.20 version from 0lento (https://github.com/0lento/UnrealEngine/tree/4.20-FleX), but he can’t make even 4.21 because of the huge changes of the engine in 4.21.
In this situation, only those who created this plugin can build a plugin for version 4.21 or 4.22. But, as I understand it, the support of this technology is not interesting, not for Nvidia, nor for Epic Games.