Pin `macos` test runners to `macos-12` #5484

Job	Run time
code-quality	1m 21s
build packages	16s
unit test / python 3.8	1m 6s
unit test / python 3.9	1m 5s
unit test / python 3.10	1m 10s
unit test / python 3.11	1m 10s
verify packages / python 3.8 / ubuntu-latest	50s
verify packages / python 3.9 / ubuntu-latest	1m 0s
verify packages / python 3.10 / ubuntu-latest	46s
verify packages / python 3.11 / ubuntu-latest	45s
verify packages / python 3.8 / macos-12	1m 13s
verify packages / python 3.9 / macos-12	1m 2s
verify packages / python 3.10 / macos-12	1m 18s
verify packages / python 3.11 / macos-12	1m 20s
verify packages / python 3.8 / windows-latest	2m 35s
verify packages / python 3.9 / windows-latest	2m 6s
verify packages / python 3.10 / windows-latest	2m 26s
verify packages / python 3.11 / windows-latest	2m 3s
	23m 32s

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