chore(3.x): release 3.8.15-SNAPSHOT (#3477) #8616

Job	Run time
parallel-integrationTests (config)	4m 56s
integrationTests (bigquery)	22m 3s
parallel-integrationTests (pubsub)	10m 19s
integrationTests (cloudsql)	21m 32s
parallel-integrationTests (pubsub-bus)	5m 10s
parallel-integrationTests (pubsub-docs)	7m 21s
parallel-integrationTests (pubsub-emulator)	6m 9s
parallel-integrationTests (pubsub-integration)	5m 33s
integrationTests (datastore)	22m 34s
parallel-integrationTests (secretmanager)	5m 28s
integrationTests (firestore)	21m 36s
parallel-integrationTests (vision)	5m 31s
integrationTests (kms)	21m 7s
integrationTests (logging)	20m 34s
integrationTests (metrics)	21m 30s
integrationTests (multisample)	20m 56s
integrationTests (kotlin)	21m 26s
integrationTests (spanner)	30m 21s
integrationTests (storage)	21m 9s
integrationTests (trace)	21m 24s
	5h 16m 39s

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