CI: Updated builds to Qt 6.8

[bjorn]

The release announcement sounds good:

For this release we have focused on improving and stabilizing existing functionality. With over 500 bug fixes and performance improvements since Qt 6.7, your existing code will run better without changing a single line.

[bjorn]

Alright, likely the install-qt.sh will need some adjustment for Qt 6.8:

Could not determine a remote URL for qtbase with version 6.8.0