ci: declare workflow-level `contents: read` on 5 workflows #780

Job	Run time
ruby-versions / Generate Ruby versions	3s
test (3.4)	4m 57s
test (3.2)	4m 6s
test (head)	3m 38s
test (4.0)	3m 33s
test (3.3)	4m 38s
	20m 55s

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