feat: Move release note generation to a sub module #4258

Job	Run time
downstream-compatibility-spring-generator	9m 9s
downstream-compatibility (google-cloud-java)	11m 48s
downstream-compatibility (java-bigtable)	4m 5s
downstream-compatibility (java-bigquery)	3m 51s
downstream-compatibility (java-bigquerystorage)	9m 58s
downstream-compatibility (java-datastore)	3m 42s
downstream-compatibility (java-firestore)	2m 19s
downstream-compatibility (java-logging)	2m 15s
downstream-compatibility (java-logging-logback)	1m 59s
downstream-compatibility (java-pubsub)	2m 31s
downstream-compatibility (java-pubsublite)	2m 24s
downstream-compatibility (java-spanner-jdbc)	3m 13s
downstream-compatibility (java-spanner)	4m 49s
downstream-compatibility (java-storage)	4m 11s
downstream-compatibility (java-storage-nio)	2m 43s
	1h 8m 57s

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