ci: Update macOS from macos-14 to macos-15 #241

Job	Run time
macos (tiny, macos-13)	1m 24s
linux (tiny, clang, native)	2m 40s
macos (tiny, macos-15)	44s
windows (HUGE, msvc, no, no, x64, stable)	6m 34s
linux (tiny, gcc, native)	2m 27s
macos (normal, macos-13)	7m 33s
windows (HUGE, mingw, yes, yes, x86, stable, yes)	12m 1s
linux (normal, clang, native)	5m 49s
macos (normal, macos-15)	4m 49s
windows (HUGE, msvc, no, yes, x86)	9m 46s
linux (normal, gcc, native)	6m 23s
macos (huge, macos-13)	18m 30s
windows (HUGE, mingw, yes, no, x64, yes)	10m 41s
linux (huge, clang, native)	11m 49s
macos (huge, macos-15)	18m 7s
windows (NORMAL, msvc, yes, no, x86)	7m 25s
linux (huge, gcc, native)	13m 29s
windows (NORMAL, mingw, no, yes, x64)	9m 32s
linux (tiny, clang, nogui)	2m 36s
windows (TINY, msvc, yes, yes, x64)	2m 8s
linux (tiny, gcc, nogui)	2m 27s
linux (normal, gcc, i386, ./src/shadow)	6m 28s
windows (TINY, mingw, no, no, x86)	2m 14s
linux (huge, gcc, uchar, testgui, true, dynamic)	11m 19s
linux (huge, clang, asan, 5.1)	24m 26s
linux (huge, gcc, unittests, true)	2m 29s
linux (normal, gcc, vimtags)	2m 9s
	3h 25m 59s

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