Qt 6.2 compatibility

[sommerluk]

In the README PlastikStyle is declared compatible with Qt 6.2. However, on my local system it does not build against 6.2.

This PR solves the build issues.

• Remove qt_standard_project_setup (only available starting with Qt 6.3)
• Substitute #include <QMutableVectorIterator> which apparently is not found.

[MartinF99]

Why did you use #include "qvector.h" instead of `#include \<QVector>?

[sommerluk]

There was no particular reason, but all the previous lines also had this form.