Change the build system to enable building with Qt6

This was built with:
-DQT_MAJOR_VERSION=6 \
-DEXCLUDE_DEPRECATED_BEFORE_AND_AT=5.90.0 \
-DKF_DISABLE_DEPRECATED_BEFORE_AND_AT=0x055a00
3 jobs for !59 with work/ahmad/qt6 in 2 minutes and 41 seconds (queued for 4 seconds)
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