Bump version, drop Qt5/KF5 support

Drop all pre-Qt6 ifdefs. This includes migrating KDE4 settings. It is safe to assume that anyone interested in keeping their Amarok 2.9 and earlier settings has used a Qt5/KF5 Amarok during these 5 years and has their old settings migrated already.

Also commented out some functionalities that are not yet there for Qt6/KF6, but that might not be essential, or will be reimplemented later: network caching, scripting code completion etc.