abandon macos-12 (GitHub's decision) and add macos-15 #1296

Job	Run time
canceller	12s
tests-Unix-CLI (ubuntu-20.04, 7, 3.8)	12m 13s
tests-Unix-CLI (ubuntu-20.04, 11, 3.9)	12m 36s
tests-Unix-GUI (ubuntu-20.04, 5.9.5, 7)	5m 4s
tests-Windows-CLI (mingw64, x86_64, 3.8)	22m 40s
tests-Unix-CLI (macos-13, 3.9)	17m 17s
tests-Windows-GUI (mingw64, x86_64)	12m 27s
tests-Unix-GUI (ubuntu-20.04, 5.15.2, 11)	6m 25s
tests-Unix-CLI (macos-15, 3.9)	9m 54s
tests-Unix-GUI (ubuntu-22.04, 5.15.2, 12)	6m 42s
tests-Windows-CLI (ucrt64, ucrt-x86_64, 3.8)	20m 43s
tests-Windows-GUI (ucrt64, ucrt-x86_64)	12m 35s
tests-Unix-GUI (ubuntu-22.04, 6.7.2, 12)	7m 14s
tests-Unix-GUI (macos-12, 5.9.5)	5m 41s
tests-Unix-GUI (macos-13, 5.15.2)	8m 10s
tests-windows-latest-pacman (mingw64, x86_64)	1m 37s
tests-Unix-GUI (macos-13, 6.7.2)	10m 9s
tests-windows-latest-pacman (ucrt64, ucrt-x86_64)	1m 37s
	2h 53m 16s

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