chore: migrate library generation IT to cloud build #5189

Job	Run time
downstream-compatibility-spring-generator	8m 32s
downstream-compatibility (google-cloud-java)	11m 46s
downstream-compatibility (java-bigtable)	4m 11s
downstream-compatibility (java-bigquery)	2m 47s
downstream-compatibility (java-bigquerystorage)	9m 23s
downstream-compatibility (java-datastore)	2m 47s
downstream-compatibility (java-firestore)	1m 58s
downstream-compatibility (java-logging)	1m 30s
downstream-compatibility (java-logging-logback)	1m 31s
downstream-compatibility (java-pubsub)	1m 47s
downstream-compatibility (java-pubsublite)	2m 1s
downstream-compatibility (java-spanner-jdbc)	2m 7s
downstream-compatibility (java-spanner)	4m 13s
downstream-compatibility (java-storage)	3m 41s
downstream-compatibility (java-storage-nio)	1m 55s
	1h 0m 9s

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