C4_LIKELY now uses [[likely]] when possible

[biojppm]

• [BREAKING] C4_LIKELY() and C4_UNLIKELY() now use [[likely]] and [[unlikely]] in C++20. Every use of this macro needs to refactored:

  if(C4_LIKELY(condition)) ... // error now
  // needs to be rewritten as:
  if C4_LIKELY(condition)  ...
  // same for C4_UNLIKELY

• Add C4_LIKELY20 and C4_UNLIKELY20, which resolve directly into [[likely]] and [[unlikely]] when C++ is 20 or later, or nothing otherwise.
• Add C++23 detection (C4_CPP and C4_CPP23)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.54%. Comparing base (ee72737) to head (2560fb1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #172   +/-   ##
=======================================
  Coverage   97.54%   97.54%           
=======================================
  Files          29       29           
  Lines        3341     3341           
=======================================
  Hits         3259     3259           
  Misses         82       82           

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