ci: declare workflow-level `contents: read` on 5 workflows #496

Job	Run time
ruby-versions / Generate Ruby versions	4s
test (3.0)	1m 4s
test (3.2)	1m 8s
test (debug)	1m 28s
test (3.1)	1m 11s
test (3.3)	1m 10s
test (4.0)	1m 10s
test (head)	1m 7s
test (3.4)	1m 10s
	9m 32s

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