Make kdesdk-kio Qt6-only

Konqueror being the only known tool which currently can display perldoc (khelpcenter would need hardcoded support added), and konqueror now getting official qt6-only, no need to maintain qt5 support with perldoc kio worker.

@teams/localization

See also sdk/kdesdk-kio!6 (merged) (Drop support for Qt5/KF5) & sdk/kdesdk-kio!7 (merged) (Rename translation catalog to kio6_perldoc to match version)