[Scala-Akka-Http] Fix deprecated `ActorMaterializer` since akka-http 2.6

Created by: s-nel

Fixes a deprecation issue in the Akka HTTP client generator for Scala. The ActorMaterializer type and constructor have been deprecated since Akka HTTP 2.6.0 and are throwing a warning.

ApiInvoker.scala:94:58
[E]      method apply in object ActorMaterializer is deprecated (since 2.6.0): Use the system wide materializer with stream attributes or configuration settings to change defaults
[E]      L94:   private implicit val materializer: ActorMaterializer = ActorMaterializer()
ApiInvoker.scala:94:38
[E]      class ActorMaterializer in package stream is deprecated (since 2.6.0): The Materializer now has all methods the ActorMaterializer used to have
[E]      L94:   private implicit val materializer: ActorMaterializer = ActorMaterializer()

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cc @clasnake @jimschubert @shijinkui @ramzimaalej @chameleon82 @Bouillie