suppress the cast warning in java okhttp-gson client (#20076) #871

Job	Run time
Build sbt/Scala client, servers (samples/client/petstore/java/okhttp-gson)	39s
Build sbt/Scala client, servers (samples/client/petstore/scalaz)	2m 50s
Build sbt/Scala client, servers (samples/client/petstore/scala-pekko)	41s
Build sbt/Scala client, servers (samples/client/petstore/scala-http4s)	38s
Build sbt/Scala client, servers (samples/server/petstore/scala-lagom-server)	5m 12s
Build sbt/Scala client, servers (samples/server/petstore/scala-play-server)	5m 49s
Build sbt/Scala client, servers (samples/server/petstore/scala-akka-http-server)	2m 10s
Build sbt/Scala client, servers (samples/server/petstore/scala-pekko-http-server)	40s
Build sbt/Scala client, servers (samples/server/petstore/scalatra)	2m 58s
Build sbt/Scala client, servers (samples/server/petstore/scala-finch)	3m 5s
Build sbt/Scala client, servers (samples/server/petstore/scala-http4s-server)	43s
Build sbt/Scala client, servers (samples/server/petstore/scala-cask)	1m 3s
	26m 28s

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