bump min ci version to 2.4 #99

Job	Run time
ruby-versions / Generate Ruby versions	4s
build (2.4 / ubuntu-latest)	18s
build (2.4 / macos-latest)	43s
build (2.5 / ubuntu-latest)	18s
build (2.5 / macos-latest)	46s
build (2.6 / ubuntu-latest)	19s
build (2.6 / macos-latest)	51s
build (2.7 / ubuntu-latest)	19s
build (2.7 / macos-latest)	1m 0s
build (3.0 / ubuntu-latest)	7s
build (3.0 / macos-latest)	11s
build (3.1 / ubuntu-latest)	13s
build (3.1 / macos-latest)	11s
build (3.2 / ubuntu-latest)	5s
build (3.2 / macos-latest)	17s
build (3.3 / ubuntu-latest)	8s
build (3.3 / macos-latest)	10s
build (head / ubuntu-latest)	11s
build (head / macos-latest)	13s
build (jruby / ubuntu-latest)	31s
build (jruby / macos-latest)	31s
build (jruby-head / ubuntu-latest)	30s
build (jruby-head / macos-latest)	43s
build (truffleruby / ubuntu-latest)	21s
build (truffleruby / macos-latest)	34s
build (truffleruby-head / ubuntu-latest)	27s
build (truffleruby-head / macos-latest)	55s
	10m 56s

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