Update CI macOS machine to `macos-14` #3409

Job	Run time
Hygiene	1m 6s
Test (macos-14, 5.1, false)	6m 4s
Test (ubuntu-22.04, 5.1, false)	3m 38s
Test (windows-2022, 4.14, false)	4m 3s
Test (ubuntu-22.04, ocaml-variants.5.1.0+options,ocaml-option-flambda, true)	2m 47s
Test (windows-2022, ocaml-variants.5.1.0+options,ocaml-option-mingw, false, windows-5.0: https://...	7m 14s
	24m 52s

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