Replace FC_QT6_MODE with FC_QT5_MODE
Turn the Qt6-as-special-case logic to Qt5-as-special-case in master by retiring FC_QT6_MODE macro and introducing FC_QT5_MODE.
We should backport this to S3_1. Just to have the same macro in master and S3_1, and avoiding extra work (and mistakes!) when porting patches between the branches.
Turn the Qt6-as-special-case logic to Qt5-as-special-case in master by retiring FC_QT6_MODE macro and introducing FC_QT5_MODE.