Upgrade `nlohmann/json` to latest release

The nlohmann/json library that we currently use for integration tests has a really large repository size (around 450MB with CMake's FetchContent), which is a problem when storing artifacts in the pipelines since CMake downloads it directly to _deps/nlohmann_json-src. To fix this, in later releases they have introduced the possibility of downloading releases directly as packaged files with:

FetchContent_Declare(json URL https://github.com/nlohmann/json/releases/download/v3.11.2/json.tar.xz)

This brings _deps/nlohmann_json-src down to 2MB. In addition, the latest 3.11.2 release is fully compatible with our current code.