Remove support for qt5, fix build warnings in qt 6.8.0 #298

Job	Run time
Code Linting / Format / Lint	7s
Windows / Build (x64, release, Qt 5.15.2, windows-2019)	55s
Windows / Build (x86, release, Qt 5.15.2, windows-2019)	1m 20s
Windows / Build (x64, release, Qt 6.4.3, windows-2019)	2m 28s
Linux / AppImage (release, Qt 5, appimage-qt5)	43s
Linux / deb (release, Qt 5, ubuntu-20_04)	44s
Linux / rpm (release, Qt 6, fedora-38)	1m 49s
Linux / AppImage (release, Qt 6, appimage-qt6)	2m 14s
Linux / deb (release, Qt 6, ubuntu-22_04)	2m 12s
Linux / snap	4m 31s
Linux / rpm (release, Qt 6, opensuse-15_5)	2m 37s
Linux / deb (release, Qt 6, ubuntu-23_10)	2m 21s
macOS / Build (debug, homebrew (qt5), macos-13)	1m 30s
macOS / Build (debug, homebrew (qt6), macos-13)	2m 28s
macOS / dmg (release, Qt 5.15.2, macos-13)	1m 24s
macOS / dmg (release, Qt 6.5.2, macos-13)	2m 17s
Windows / Unified x64-x86 Installer	0s
	29m 40s

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