Improve handling of pekko versions in scala-akka-http-server generato… #882

Job	Run time
Build sbt/Scala client, servers (samples/client/petstore/java/okhttp-gson)	36s
Build sbt/Scala client, servers (samples/client/petstore/scalaz)	2m 46s
Build sbt/Scala client, servers (samples/client/petstore/scala-pekko)	44s
Build sbt/Scala client, servers (samples/client/petstore/scala-http4s)	44s
Build sbt/Scala client, servers (samples/server/petstore/scala-lagom-server)	3m 51s
Build sbt/Scala client, servers (samples/server/petstore/scala-play-server)	4m 28s
Build sbt/Scala client, servers (samples/server/petstore/scala-akka-http-server)	2m 4s
Build sbt/Scala client, servers (samples/server/petstore/scala-pekko-http-server)	43s
Build sbt/Scala client, servers (samples/server/petstore/scalatra)	3m 33s
Build sbt/Scala client, servers (samples/server/petstore/scala-finch)	2m 57s
Build sbt/Scala client, servers (samples/server/petstore/scala-http4s-server)	47s
Build sbt/Scala client, servers (samples/server/petstore/scala-cask)	1m 19s
	24m 32s

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