Set MacOS platform version in one place #2621

Job	Run time
Lint bash, docker, markdown, and yaml	9m 35s
Validate GNU Docker image builds	51s
Validate GNU Docker release image builds	55s
Validate musl Docker image builds	28s
Validate musl Docker release image builds	26s
Validate Windows Docker image builds	2m 22s
Validate Windows Docker release image builds	2m 30s
armhf Linux glibc	1h 43m 12s
arm Linux glibc	1h 44m 11s
riscv64 Linux glibc	1h 44m 19s
Verify CHANGELOG is valid	10s
x86-64 Linux glibc	1h 51m 51s
x86-64 Linux musl	2h 10m 18s
x86-64 Apple Darwin	42m 47s
aarch64 Linux glibc	1h 33m 32s
x86-64 Windows MSVC	2h 47m 31s
use dtrace	24m 17s
use pool_memalign	35m 57s
	15h 35m 12s

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