Bump actions/checkout from 3 to 4 #28

Job	Run time
ruby-versions / Generate Ruby versions	2s
build (2.5 / ubuntu-latest)	23s
build (2.5 / macos-latest)	1m 14s
build (2.5 / windows-latest)	3m 21s
build (2.6 / ubuntu-latest)	30s
build (2.6 / macos-latest)	54s
build (2.6 / windows-latest)	3m 51s
build (2.7 / ubuntu-latest)	30s
build (2.7 / macos-latest)	59s
build (2.7 / windows-latest)	2m 32s
build (3.0 / ubuntu-latest)	9s
build (3.0 / macos-latest)	17s
build (3.0 / windows-latest)	1m 43s
build (3.1 / ubuntu-latest)	8s
build (3.1 / macos-latest)	1m 10s
build (3.1 / windows-latest)	1m 41s
build (3.2 / ubuntu-latest)	5s
build (3.2 / macos-latest)	1m 33s
build (3.2 / windows-latest)	2m 17s
build (head / ubuntu-latest)	9s
build (head / macos-latest)	15s
build (head / windows-latest)	59s
build (jruby / ubuntu-latest)	26s
build (jruby / macos-latest)	45s
build (jruby / windows-latest)	2m 13s
build (jruby-head / ubuntu-latest)	28s
build (jruby-head / macos-latest)	1m 40s
build (jruby-head / windows-latest)	1m 28s
build (truffleruby / ubuntu-latest)	26s
build (truffleruby / macos-latest)	58s
build (truffleruby-head / ubuntu-latest)	19s
build (truffleruby-head / macos-latest)	48s
	34m 13s

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