Default to kf6-qt6 for kdesrc-build and wiki

I am absolutely brand new to KDE development and I followed these instructions (from the wiki) and I have already run into two big errors.

First I reported as a bug report here: 476287 – kio-extras fails to compile due to missing qcoro-qt6 dependency

Second I just encountered with a failure to build systemsettings, getting

/home/ben/kde/src/systemsettings/core/ModuleView.cpp:38:10: fatal error: Kirigami/Units: No such file or directory

Searching around led me here: System settings fails to build Kirigami/Units: No such file or directory

One reason for posting this here rather than on a dedicated topic is that I have essentially a “virgin” system that has not been used for any KDE development prior to yesterday.

Edit: disregard. Nate fixed here: core: Rework ModuleView code using Kirigami Platform's Units (!263) · Merge requests · Plasma / System Settings · GitLab