chore: migrate library generation IT to cloud build #5210

Job	Run time
downstream-compatibility-spring-generator	9m 17s
downstream-compatibility (google-cloud-java)	11m 35s
downstream-compatibility (java-bigtable)	3m 46s
downstream-compatibility (java-bigquery)	2m 56s
downstream-compatibility (java-bigquerystorage)	9m 47s
downstream-compatibility (java-datastore)	3m 18s
downstream-compatibility (java-firestore)	1m 59s
downstream-compatibility (java-logging)	1m 53s
downstream-compatibility (java-logging-logback)	1m 25s
downstream-compatibility (java-pubsub)	2m 5s
downstream-compatibility (java-pubsublite)	2m 8s
downstream-compatibility (java-spanner-jdbc)	2m 17s
downstream-compatibility (java-spanner)	5m 31s
downstream-compatibility (java-storage)	3m 46s
downstream-compatibility (java-storage-nio)	2m 18s
	1h 4m 1s

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