Bump NlohmannJSON to version 3.12.0

[flubshi]

Changes

This changes NlohmannJSON from 3.7.3 to 3.12.0.

Reason

Compiling pvr.plutotv (latest Piers commit) with NlohmannJSON version 3.7.3 returns:

CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

Compiling using NlohmannJSON version 3.12.0 works fine for me.

Is there a reason to use 3.7.3 instead of latest release 3.12.0?
Or can we just bump the version?
I runtime tested with 3.12.0 and everything seems to work (EPG, Channel playback).

[ksooo]

Is there a reason to use 3.7.3 instead of latest release 3.12.0?

3.12.0 does not compile on tvOS. At least the tests do not.

=> https://jenkins.kodi.tv/blue/rest/organizations/jenkins/pipelines/kodi-pvr/pipelines/pvr.plutotv/branches/PR-130/runs/1/nodes/260/log/?start=0