remove Ubuntu 20.04 test with Qt6; not a supported platform #1206

Job	Run time
canceller	4s
tests-Unix-CLI (ubuntu-20.04, 7, 3.8)	5m 11s
tests-Unix-GUI (ubuntu-20.04, 5.9.5, 7)	4m 39s
tests-Windows-CLI (mingw64, x86_64, 3.8)	5m 5s
tests-Unix-CLI (ubuntu-20.04, 11, 3.9)	5m 12s
tests-Unix-GUI (ubuntu-20.04, 5.15.2, 11)	5m 11s
tests-Windows-CLI (ucrt64, ucrt-x86_64, 3.8)	5m 5s
tests-Unix-CLI (macos-12, 3.8)	5m 12s
tests-Windows-GUI (mingw64, x86_64)	5m 3s
tests-Unix-GUI (ubuntu-22.04, 5.15.2, 12)	5m 11s
tests-Unix-CLI (macos-13, 3.9)	5m 12s
tests-Windows-GUI (ucrt64, ucrt-x86_64)	5m 4s
tests-Unix-GUI (ubuntu-22.04, 6.7.2, 12)	3m 29s
tests-windows-latest-pacman (mingw64, x86_64)	1m 51s
tests-Unix-GUI (macos-12, 5.9.5)	5m 12s
tests-windows-latest-pacman (ucrt64, ucrt-x86_64)	1m 43s
tests-Unix-GUI (macos-13, 5.15.2)	5m 11s
tests-Unix-GUI (macos-13, 6.7.2)	5m 10s
	1h 18m 45s

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