cxx-qt-lib: qdatetime put deprecated behind cfg flag

[ahayzen-kdab]

Closes #1106

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (fd558d5) to head (456d609). Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1109   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           71        71           
  Lines        11949     11949           
=========================================
  Hits         11949     11949           

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