ci: Remove old MacOS-12 CI test #6400

Job	Run time
VFX2021 gcc9/C++17 py3.7 exr3.1 ocio2.0	13m 22s
macos-13 appleclang14/C++17 py3.11 MacOS-13	14m 57s
VFX2021 clang10/C++17 avx2 exr3.1 ocio2.0	13m 30s
macos-14 appleclang15/C++20 py3.12 MacOS-14-ARM	10m 57s
VFX2022 gcc9/C++17 py39 exr3.1 ocio2.1	12m 30s
VFX2022 clang13/C++17 py39 avx2 exr3.1 ocio2.1	12m 34s
VFX2022 icc/C++17 py3.9 exr3.1 ocio2.1 qt5.15	10m 25s
VFX2022 icx/C++17 py3.9 exr3.1 ocio2.2 qt5.15	15m 30s
Ubuntu latest releases gcc13 C++20 py3.12 avx2 exr3.2 ocio2.3	19m 5s
VFX2024 sanitizers	33m 53s
Ubuntu bleeding edge gcc14 C++20 py3.12 OCIO/libtiff/exr-main avx2	7m 10s
VFX2023 gcc11/C++17 py3.10 exr3.1 ocio2.2	11m 31s
Ubuntu all local builds gcc12 C++17 avx2 exr3.2 ocio2.3	17m 49s
VFX2024 gcc11/C++17 py3.11 exr3.2 ocio2.3	13m 25s
Ubuntu clang14 C++20 avx2 exr3.1 ocio2.1	13m 39s
VFX2021 oldest gcc9.3/C++17 py3.7 exr-3.1	16m 15s
Ubuntu debug gcc9/C++17, sse4.2, exr3.1	16m 51s
VFX2021 hobbled gcc9.3/C++17 py3.7 exr-3.1 no-sse	16m 20s
Ubuntu static libs gcc9 C++17 exr2.4	13m 39s
VFX2023 abi check	14m 46s
Ubuntu clang-format	6m 16s
windows-2019 VS2019	51m 10s
windows-2022 VS2022	48m 58s
	6h 44m 32s

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