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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