Remove support for qt5, fix build warnings in qt 6.8.0 #304

Job	Run time
Code Linting / Format / Lint	12s
Linux / snap	10m 58s
Linux / AppImage (release, Qt 5, appimage-qt5)	36s
Linux / deb (release, Qt 5, ubuntu-20_04)	25s
Linux / AppImage (release, Qt 6, appimage-qt6)	2m 13s
Linux / rpm (release, Qt 6, fedora-38)	1m 51s
macOS / Build (debug, homebrew (qt5), macos-13)	1m 52s
Linux / deb (release, Qt 6, ubuntu-22_04)	2m 15s
Linux / rpm (release, Qt 6, opensuse-15_5)	2m 35s
macOS / Build (debug, homebrew (qt6), macos-13)	3m 32s
Linux / deb (release, Qt 6, ubuntu-23_10)	2m 12s
Windows / Build (x64, release, Qt 5.15.2, windows-2019)	50s
Windows / Build (x86, release, Qt 5.15.2, windows-2019)	50s
macOS / dmg (release, Qt 5.15.2, macos-13)	43s
Windows / Build (x64, release, Qt 6.4.3, windows-2019)	2m 26s
macOS / dmg (release, Qt 6.5.2, macos-13)	8m 50s
Windows / Unified x64-x86 Installer	0s
	42m 20s

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