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

All unit tests pass.
3 jobs for !18 with work/ahmad/qt6 in 1 minute and 19 seconds (queued for 11 seconds)
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