Make ksirk & libkdegames Qt6-only
Ksirk was the last consumer of libkdegames/Qt5 on master, so libkdegames can be switched now as well.
(libkmahjongg still waiting on kajongg for sorting out qt6-only)
For libkdegames I propose afterwards to rename the catalogs from libkdegames5 to libkdegames6. Might allow some people to still co-install older libkdegames/Qt5