[MNT] CI: Bump PyQt6 versions in the test matrix

[ales-erjavec]

• Bump PyQt6 versions in the test matrix (in particular include PyQt6 6.11)
• Also bump gh actions versions to avoid node 20 deprecation warnings in CI logs

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.34%. Comparing base (88cd975) to head (f53b387).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #350      +/-   ##
==========================================
+ Coverage   76.30%   76.34%   +0.03%     
==========================================
  Files         102      102              
  Lines       21363    21363              
==========================================
+ Hits        16302    16309       +7     
+ Misses       5061     5054       -7     

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