Update PawnPP for the C++20 requirement

Picks up "require C++20 for the signed shift guarantee", which raises
the pawnpp interface target from cxx_std_17 to cxx_std_20 because
amx.h's OP_SSHR relies on P0907 to make a negative right shift an
arithmetic shift rather than implementation-defined.

The driver build is not affected either way. The root CMakeLists
defines its own PawnPP INTERFACE target for the include directory and
never adds PawnPP/CMakeLists.txt as a subdirectory, and it already sets
CMAKE_CXX_STANDARD 20. This only reaches consumers that build PawnPP as
its own project.