Refactor error codes, conditions and remove generic_category alias

[grisumbras]

• Fix Remove condition::assign_error #756
• Fix Remove json::generic_category #758

[cppalliance-bot]

An automated preview of the documentation is available at https://778.jsondocs.prtest.cppalliance.org/libs/json/doc/html/index.html

[codecov]

Codecov Report

Merging #778 (fde4e87) into develop (f4031f3) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #778      +/-   ##
===========================================
- Coverage    98.99%   98.98%   -0.01%     
===========================================
  Files           70       70              
  Lines         6734     6732       -2     
===========================================
- Hits          6666     6664       -2     
  Misses          68       68              

Impacted Files	Coverage Δ
include/boost/json/detail/value_from.hpp	100.00% <ø> (ø)
include/boost/json/detail/value_to.hpp	100.00% <ø> (ø)
include/boost/json/impl/conversion.hpp	100.00% <ø> (ø)
include/boost/json/system_error.hpp	100.00% <ø> (ø)
include/boost/json/impl/error.ipp	100.00% <100.00%> (ø)

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[cppalliance-bot]

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