feat: Move release note generation to a sub module #4256

Job	Run time
downstream-compatibility-spring-generator	9m 4s
downstream-compatibility (google-cloud-java)	13m 4s
downstream-compatibility (java-bigtable)	4m 59s
downstream-compatibility (java-bigquery)	3m 15s
downstream-compatibility (java-bigquerystorage)	9m 49s
downstream-compatibility (java-datastore)	3m 1s
downstream-compatibility (java-firestore)	2m 53s
downstream-compatibility (java-logging)	1m 58s
downstream-compatibility (java-logging-logback)	1m 22s
downstream-compatibility (java-pubsub)	2m 43s
downstream-compatibility (java-pubsublite)	2m 22s
downstream-compatibility (java-spanner-jdbc)	3m 27s
downstream-compatibility (java-spanner)	5m 12s
downstream-compatibility (java-storage)	4m 2s
downstream-compatibility (java-storage-nio)	2m 2s
	1h 9m 13s

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