[wpiutil] Update json implementation

[PeterJohnson]

The current JSON implementation in wpiutil is based on a very old version of nlohmann JSON with lots of customizations to reduce compile time overhead (untemplating, using LLVM containers, moving a lot of code out of line). This should be updated to the most recent version. Ideally it would also use upstream_utils, but this is likely quite difficult due to the number of customizations.

[sciencewhiz]

On Ubuntu 2023.04 with gcc 12.2, the JsonComparisonTypesTest Less test fails. It passes if gcc is downgraded to GCC 11.3. This appears to have been fixed upstream in nlohmann/json#3446

[PeterJohnson]

Fixed by #5680.