There are two prison build artifacts being produced. One for legacy spigot and paper releases. And for the newer spigot 26 release, which includes paper 26 too.

prison-3.3.1.jar - Supports Spigot 1.8+ through to 1.20+
- works with spigot 1.21+ too, but without newer block support

prison26-3.3.1.jar - Supports Spigot 1.21+, Spigot 26, and Paper 26
- supports the latest blocks