fix: upgrade nlohmann/json to v3.12.0 for CMake 4 compatibility

[BeiBaiNian]

Why

The Windows CI job (build (windows-latest, 3.12) in python_binding.yml) fails at the "Build and install" step with:

CMake Error at build/temp.win-amd64-cpython-312/Release/_deps/json-src/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

The Windows runner ships a recent CMake (4.x), which removed compatibility with cmake_minimum_required(VERSION < 3.5). The pinned nlohmann/json v3.11.2 declares cmake_minimum_required(VERSION 3.1), so CMake 4.x rejects it while configuring the FetchContent dependency. The Ubuntu job is unaffected because its CMake is still 3.x.

What changed

• cmake/modules/Findjson.cmake: bump the FetchContent GIT_TAG of nlohmann/json from v3.11.2 to v3.12.0. v3.12.0 declares cmake_minimum_required(VERSION 3.5...4.0), which is accepted by CMake 4.x.

Verification

• Reproduced the CI failure locally with CMake 4.4.2 + json v3.11.2: same error as the Windows CI log.
• With json v3.12.0 + CMake 4.4.2: cmake configure succeeds, and mingw32-make casbin builds libcasbin.a successfully.
• Only 1 file changed; the fix is limited to the dependency version bump.