Improve handling of pekko versions in scala-akka-http-server generator #880

Job	Run time
Build sbt/Scala client, servers (samples/client/petstore/java/okhttp-gson)	44s
Build sbt/Scala client, servers (samples/client/petstore/scalaz)	1m 55s
Build sbt/Scala client, servers (samples/client/petstore/scala-pekko)	41s
Build sbt/Scala client, servers (samples/client/petstore/scala-http4s)	44s
Build sbt/Scala client, servers (samples/server/petstore/scala-lagom-server)	4m 3s
Build sbt/Scala client, servers (samples/server/petstore/scala-play-server)	4m 13s
Build sbt/Scala client, servers (samples/server/petstore/scala-akka-http-server)	44s
Build sbt/Scala client, servers (samples/server/petstore/scala-pekko-http-server)	43s
Build sbt/Scala client, servers (samples/server/petstore/scalatra)	2m 29s
Build sbt/Scala client, servers (samples/server/petstore/scala-finch)	3m 14s
Build sbt/Scala client, servers (samples/server/petstore/scala-http4s-server)	49s
Build sbt/Scala client, servers (samples/server/petstore/scala-cask)	1m 17s
	21m 36s

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