Bump actions/checkout from 3 to 4 #371

Job	Run time
ruby-versions / Generate Ruby versions	2s
test (2.5, ubuntu-latest)	36s
test (2.5, macos-latest)	1m 38s
test (2.5, windows-latest)	4m 15s
test (2.6, ubuntu-latest)	38s
test (2.6, macos-latest)	1m 36s
test (2.6, windows-latest)	4m 8s
test (2.7, ubuntu-latest)	39s
test (2.7, macos-latest)	1m 7s
test (2.7, windows-latest)	2m 18s
test (3.0, ubuntu-latest)	22s
test (3.0, macos-latest)	37s
test (3.0, windows-latest)	2m 0s
test (3.1, ubuntu-latest)	21s
test (3.1, macos-latest)	35s
test (3.1, windows-latest)	2m 43s
test (3.2, ubuntu-latest)	34s
test (3.2, macos-latest)	1m 20s
test (3.2, windows-latest)	2m 57s
test (head, ubuntu-latest)	28s
test (head, macos-latest)	2m 9s
test (head, windows-latest)	1m 44s
test (jruby, ubuntu-latest)	6m 31s
test (jruby-head, ubuntu-latest)	7m 54s
test (truffleruby, ubuntu-latest)	2m 6s
test (truffleruby-head, ubuntu-latest)	4m 20s
	53m 38s

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