Clean up for Qt 6.10

[BSDKaffee]

Clean up after e061ca9 and 1039035 because of e7dd572. So, e7dd572 should have included a Qt version check. e061ca9 adds this check and fixes build with Qt <= 6.9, but there is no QT_VERSION variable defined in the Qt CMake configs. This again breaks Qt >= 6.10. So, 1039035 does some creative things to make the build work again with Qt 6.10, but this is misguided and non-standard. All that needs be done is to compare Qt6_VERSION (case sensitive) in src/platform/x11/x11platform.cmake.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.90%. Comparing base (7934ae6) to head (4c48dc3).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3339   +/-   ##
=======================================
  Coverage   79.89%   79.90%           
=======================================
  Files         231      231           
  Lines       28469    28469           
=======================================
+ Hits        22746    22747    +1     
+ Misses       5723     5722    -1     

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[hluk]

Thank you!