Update actions/checkout action to v6 #2178

Job	Run time
check	4m 50s
Docker CLI tests (ubuntu-latest)	5m 51s
Tests (ubuntu-latest, 17)	10m 59s
bazel_aspect	2m 52s
Tests (ubuntu-latest, 8)	13m 59s
Maven tests (8)	2m 31s
bazel	1m 23s
Tests (ubuntu-latest, 11)	13m 43s
Maven tests (11)	2m 45s
Tests (ubuntu-latest, 21)	7m 24s
Maven tests (17)	2m 37s
Maven tests (21)	2m 21s
	1h 11m 15s

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