Remove support for qt5, fix build warnings in qt 6.8.0 #305

Job	Run time
Code Linting / Format / Lint	9s
Linux / deb (release, Qt 6, ubuntu-22_04)	1m 59s
Linux / deb (release, Qt 6, ubuntu-23_10)	2m 13s
Linux / rpm (release, Qt 6, fedora-38)	1m 51s
Linux / snap	4m 49s
Linux / AppImage (release, Qt 6, appimage-qt6)	2m 8s
Linux / rpm (release, Qt 6, opensuse-15_5)	2m 43s
Windows / Build (release, Qt 6.4.3, windows-2019)	2m 29s
macOS / Build (debug, homebrew (qt6), macos-13)	2m 53s
macOS / dmg (release, Qt 6.5.2, macos-13)	5m 31s
Windows / x64 Installer	45s
	27m 30s

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