shift to Qt 5.15.0 minimum #1197

Job	Run time
canceller	2s
tests-Unix-GUI (ubuntu-20.04, 5.9.5, 7)	4m 51s
tests-Windows-CLI (mingw64, x86_64, 3.8)	20m 41s
tests-Unix-GUI (ubuntu-20.04, 5.15.2, 11)	5m 37s
tests-Windows-CLI (ucrt64, ucrt-x86_64, 3.8)	20m 19s
tests-Unix-CLI (ubuntu-20.04, 7, 3.8)	9m 18s
tests-Unix-GUI (ubuntu-22.04, 5.15.2, 12)	6m 16s
tests-Unix-CLI (ubuntu-20.04, 11, 3.9)	10m 3s
tests-Unix-GUI (macos-12, 5.9.5)	5m 31s
tests-Unix-CLI (macos-12, 3.8)	17m 42s
tests-Unix-GUI (macos-13, 5.15.2)	7m 11s
tests-Windows-GUI (mingw64, x86_64)	12m 14s
tests-windows-latest-pacman (mingw64, x86_64)	1m 41s
tests-Unix-CLI (macos-13, 3.9)	15m 40s
tests-Windows-GUI (ucrt64, ucrt-x86_64)	12m 10s
tests-windows-latest-pacman (ucrt64, ucrt-x86_64)	1m 40s
	2h 30m 56s

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