github: move to Qt 6.9.0 for CI tests

[ahayzen-kdab]

Requires #1257

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (fef1832) to head (ae16f3e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1258   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           73        73           
  Lines        12634     12634           
=========================================
  Hits         12634     12634           

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