Export APSI::apsi with the JsonCpp::JsonCpp target

[BillyONeal]

APSI's exported APSI::apsi target links the bare CMake target jsoncpp_static. With vcpkg's dynamic jsoncpp build, that target is not defined; the package provides the stable JsonCpp::JsonCpp target instead. As a result, a downstream find_package(APSI CONFIG REQUIRED) consumer configures but fails to link with jsoncpp_static.lib not found.

Replacing jsoncpp_static with JsonCpp::JsonCpp in target_link_libraries(apsi ...) fixes both Release and Debug consumers while continuing to work with jsoncpp's installed CMake package.

This was found while validating microsoft/vcpkg#53313.

[BillyONeal]

GPT 5.6 Sol wrote the above; I did not ask it to do that. But it isn't wrong :)