chore(3.x): release 3.8.15 #8639

Job	Run time
integrationTests (bigquery)	12m 50s
parallel-integrationTests (config)	4m 58s
integrationTests (cloudsql)	12m 26s
parallel-integrationTests (pubsub)	11m 17s
integrationTests (datastore)	15m 1s
parallel-integrationTests (pubsub-bus)	5m 6s
integrationTests (firestore)	12m 32s
integrationTests (kms)	12m 16s
parallel-integrationTests (pubsub-docs)	7m 29s
integrationTests (logging)	12m 14s
parallel-integrationTests (pubsub-emulator)	5m 18s
integrationTests (metrics)	12m 56s
parallel-integrationTests (pubsub-integration)	5m 9s
integrationTests (multisample)	12m 11s
parallel-integrationTests (secretmanager)	5m 11s
integrationTests (kotlin)	12m 5s
parallel-integrationTests (vision)	5m 33s
integrationTests (spanner)	18m 54s
integrationTests (storage)	12m 11s
integrationTests (trace)	12m 41s
	3h 28m 18s

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