Revert "Switch to macos-13 in CI" #155

Job	Run time
ruby-versions / Generate Ruby versions	3s
build (2.5 / ubuntu-latest)	20s
build (2.5 / macos-latest)	51s
build (2.5 / windows-latest)	2m 19s
build (2.6 / ubuntu-latest)	23s
build (2.6 / macos-latest)	59s
build (2.6 / windows-latest)	3m 6s
build (2.7 / ubuntu-latest)	26s
build (2.7 / macos-latest)	56s
build (2.7 / windows-latest)	2m 5s
build (3.0 / ubuntu-latest)	11s
build (3.0 / macos-latest)	28s
build (3.0 / windows-latest)	1m 42s
build (3.1 / ubuntu-latest)	12s
build (3.1 / macos-latest)	21s
build (3.1 / windows-latest)	1m 37s
build (3.2 / ubuntu-latest)	10s
build (3.2 / macos-latest)	19s
build (3.2 / windows-latest)	1m 28s
build (3.3 / ubuntu-latest)	11s
build (3.3 / macos-latest)	21s
build (3.3 / windows-latest)	2m 27s
build (head / ubuntu-latest)	13s
build (head / macos-latest)	21s
build (truffleruby-head / ubuntu-latest)	26s
build (truffleruby-head / macos-latest)	52s
build (mswin / windows-latest)	1m 35s
build (mingw / windows-latest)	1m 28s
	25m 50s

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