Nlohmannjson: Update to version 3.11.2

Nlohmannjson: Update to version 3.11.2

Compile tested: OpenWRT-23.05-rc2, arm_cortex-a7_neon-vfpv4, mips_24kc
Run tested: arm_cortex-a7_neon-vfpv4 (Linksys MR8300), mips_24kc (tplink_archer-a7), OpenWrt 23.05-rc2)

* Update package nlohmannjson to version 3.11.2.
* Version 3.11.2 is source compatible with 3.10.2 so no package should fail to compile
* From version 3.11.2 on forward template declarations are collected in the file
  json_fwd.hpp. The json.hpp still contains all template definitions.
  Because in most compilation units of a software package only the file json_fwd.hpp
  needs to be included, the compilation process is accelerated a lot.

Signed-off-by: Volker Christian <redacted>