Don't use deprecated form of user-defined literal operator

[Thomas-Barbier-1A]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.1%. Comparing base (49ef290) to head (ade2b95).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #663     +/-   ##
=========================================
- Coverage     94.1%   94.1%   -0.0%     
=========================================
  Files          279     279             
  Lines        28979   28979             
=========================================
- Hits         27255   27253      -2     
- Misses        1724    1726      +2     

Files with missing lines	Coverage Δ
include/boost/multiprecision/cpp_int/literals.hpp	100.0% <ø> (ø)

... and 1 file with indirect coverage changes

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[ckormanyos]

Thanks @Thomas-Barbier-1A for catching this and @mborland for the quick fix for the next release!