Remove support for Qt6 versions less than 6.3 #132

Job	Run time
build (ubuntu-latest, ci-dev-client-only-qt5, 5.15)	3m 27s
build (ubuntu-latest, ci-dev-client-only-qt6, 6.5.*, qtshadertools qtscxml)	5m 1s
build (ubuntu-latest, ci-dev-client-and-ui-qt5, 5.15)	6m 9s
build (ubuntu-latest, ci-dev-client-and-ui-qt6, 6.5.*)	7m 40s
build (windows-latest, ci-dev-client-only-qt5, 5.15)	3m 53s
build (windows-latest, ci-dev-client-only-qt6, 6.5.*, qtshadertools qtscxml)	7m 8s
build (windows-latest, ci-dev-client-and-ui-qt5, 5.15)	7m 31s
build (windows-latest, ci-dev-client-and-ui-qt6, 6.5.*)	10m 47s
build (macos-latest, ci-dev-client-only-qt5, 5.15)	8m 27s
build (macos-latest, ci-dev-client-only-qt6, 6.5.*, qtshadertools qtscxml)	9m 1s
build (macos-latest, ci-dev-client-and-ui-qt5, 5.15)	7m 55s
build (macos-latest, ci-dev-client-and-ui-qt6, 6.5.*)	10m 33s
	1h 27m 32s

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