ci: Update macOS from macos-14 to macos-15 #242

Job	Run time
macos (tiny, macos-13)	1m 9s
linux (tiny, clang, native)	3m 13s
macos (tiny, macos-15)	1m 9s
windows (HUGE, msvc, no, no, x64, stable)	6m 52s
linux (tiny, gcc, native)	4m 54s
macos (normal, macos-13)	8m 30s
windows (HUGE, mingw, yes, yes, x86, stable, yes)	13m 3s
linux (normal, clang, native)	6m 9s
macos (normal, macos-15)	5m 1s
windows (HUGE, msvc, no, yes, x86)	10m 14s
linux (normal, gcc, native)	9m 35s
macos (huge, macos-13)	21m 11s
windows (HUGE, mingw, yes, no, x64, yes)	10m 46s
linux (huge, clang, native)	12m 49s
macos (huge, macos-15)	17m 43s
windows (NORMAL, msvc, yes, no, x86)	8m 27s
linux (huge, gcc, native)	17m 1s
linux (tiny, clang, nogui)	2m 42s
windows (NORMAL, mingw, no, yes, x64)	9m 56s
linux (tiny, gcc, nogui)	5m 1s
windows (TINY, msvc, yes, yes, x64)	1m 47s
linux (normal, gcc, i386, ./src/shadow)	9m 56s
windows (TINY, mingw, no, no, x86)	2m 41s
linux (huge, gcc, uchar, testgui, true, dynamic)	14m 41s
linux (huge, clang, asan, 5.1)	26m 1s
linux (huge, gcc, unittests, true)	5m 48s
linux (normal, gcc, vimtags)	6m 30s
	4h 2m 49s

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