CI: Remove wrongly-added Ruby 2.4 #110

Job	Run time
ruby-versions / Generate Ruby versions	3s
build (2.5 / ubuntu-latest)	19s
build (2.5 / windows-latest)	3m 5s
build (2.6 / ubuntu-latest)	18s
build (2.6 / macos-latest)	25s
build (2.6 / windows-latest)	3m 12s
build (2.7 / ubuntu-latest)	18s
build (2.7 / macos-latest)	29s
build (2.7 / windows-latest)	2m 31s
build (3.0 / ubuntu-latest)	5s
build (3.0 / macos-latest)	8s
build (3.0 / windows-latest)	49s
build (3.1 / ubuntu-latest)	7s
build (3.1 / macos-latest)	8s
build (3.1 / windows-latest)	55s
build (3.2 / ubuntu-latest)	8s
build (3.2 / macos-latest)	11s
build (3.2 / windows-latest)	51s
build (3.3 / ubuntu-latest)	8s
build (3.3 / macos-latest)	8s
build (3.3 / windows-latest)	2m 18s
build (head / ubuntu-latest)	8s
build (head / macos-latest)	9s
build (head / windows-latest)	57s
build (2.5 / macos-13)	50s
	18m 40s

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