ci: Update macOS from macos-14 to macos-15 #243

Job	Run time
linux (tiny, clang, native)	2m 11s
linux (tiny, gcc, native)	5m 24s
linux (normal, clang, native)	6m 30s
macos (tiny, macos-13)	1m 12s
windows (HUGE, msvc, no, no, x64, stable)	6m 24s
linux (normal, gcc, native)	9m 24s
macos (tiny, macos-15)	48s
windows (HUGE, mingw, yes, yes, x86, stable, yes)	12m 44s
macos (normal, macos-13)	7m 52s
linux (huge, clang, native)	11m 56s
windows (HUGE, msvc, no, yes, x86)	8m 35s
macos (normal, macos-15)	5m 35s
linux (huge, gcc, native)	17m 2s
linux (tiny, clang, nogui)	2m 22s
macos (huge, macos-13)	20m 47s
windows (HUGE, mingw, yes, no, x64, yes)	10m 5s
linux (tiny, gcc, nogui)	4m 43s
windows (NORMAL, msvc, yes, no, x86)	8m 0s
macos (huge, macos-15)	17m 45s
linux (normal, gcc, i386, ./src/shadow)	12m 42s
windows (NORMAL, mingw, no, yes, x64)	9m 37s
linux (huge, gcc, uchar, testgui, true, dynamic)	14m 34s
windows (TINY, msvc, yes, yes, x64)	1m 50s
linux (huge, clang, asan, 5.1)	26m 17s
windows (TINY, mingw, no, no, x86)	2m 33s
linux (huge, gcc, unittests, true)	6m 18s
linux (normal, gcc, vimtags)	5m 44s
	3h 58m 54s

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