use C++17 for Qt6 builds in CMake #1207

Job	Run time
canceller	4s
tests-Windows-CLI (mingw64, x86_64, 3.8)	20m 3s
tests-Unix-CLI (ubuntu-20.04, 7, 3.8)	9m 17s
tests-Windows-CLI (ucrt64, ucrt-x86_64, 3.8)	20m 56s
tests-Unix-GUI (ubuntu-20.04, 5.9.5, 7)	4m 41s
tests-Unix-CLI (ubuntu-20.04, 11, 3.9)	10m 50s
tests-Windows-GUI (mingw64, x86_64)	12m 46s
tests-Unix-GUI (ubuntu-20.04, 5.15.2, 11)	6m 49s
tests-Unix-CLI (macos-12, 3.8)	14m 38s
tests-Windows-GUI (ucrt64, ucrt-x86_64)	12m 17s
tests-windows-latest-pacman (mingw64, x86_64)	1m 48s
tests-Unix-GUI (ubuntu-22.04, 5.15.2, 12)	6m 17s
tests-Unix-CLI (macos-13, 3.9)	12m 58s
tests-windows-latest-pacman (ucrt64, ucrt-x86_64)	1m 40s
tests-Unix-GUI (ubuntu-22.04, 6.7.2, 12)	7m 6s
tests-Unix-GUI (macos-12, 5.9.5)	7m 59s
tests-Unix-GUI (macos-13, 5.15.2)	6m 19s
tests-Unix-GUI (macos-13, 6.7.2)	12m 7s
	2h 48m 35s

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