CI: Use GH Actions checkout@v4 #429

Job	Run time
other (ubuntu, jruby-9.3)	43s
head-versions (ubuntu, ruby-head)	1m 29s
other (ubuntu, jruby-9.4)	52s
mri (ubuntu, 2.5)	33s
head-versions (ubuntu, jruby-head)	48s
valgrind	4m 48s
other (ubuntu, truffleruby)	2m 41s
mri (ubuntu, 2.6)	38s
mri (ubuntu, 2.7)	37s
mri (ubuntu, 3.0)	29s
mri (ubuntu, 3.1)	33s
mri (ubuntu, 3.2)	41s
mri (macos, 2.5)	6s
mri (macos, 3.2)	34s
mri (windows, 2.5)	3m 41s
mri (windows, 3.2)	2m 49s
	22m 2s

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