Conformance Tests

Update sigstore/sigstore-conformance action to v0.0.16 #2288

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Run time

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Job	Run time
conformance (11, production)	18m 8s
conformance (11, staging)	3m 2s
conformance (17, production)	16m 25s
conformance (17, staging)	2m 54s
	40m 29s