Replace macos-12 runners with macos-15 #250

Job	Run time
posix (clang, clang++-12, 03,11,14,17,20, ubuntu-22.04, clang-12)	10m 46s
posix (clang, clang++-13, 03,11,14,17,20, ubuntu-22.04, clang-13)	10m 45s
posix (clang, clang++-14, 03,11,14,17,20, ubuntu-22.04, clang-14)	10m 44s
posix (clang, clang++-14, 03-gnu,11-gnu,14-gnu,17-gnu,20-gnu, ubuntu-22.04, clang-14)	10m 45s
posix (clang, clang++-15, 03,11,14,17,20, ubuntu-22.04, clang-15, deb http://apt.llvm.org/jammy/ ...	10m 50s
posix (clang, clang++-16, 03,11,14,17,20, ubuntu-22.04, clang-16, deb http://apt.llvm.org/jammy/ ...	11m 8s
posix (clang, clang++-17, 03,11,14,17,20, ubuntu-22.04, clang-17, deb http://apt.llvm.org/jammy/ ...	10m 58s
	1h 15m 56s

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