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()

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
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  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
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cc @clasnake @jimschubert @shijinkui @ramzimaalej @chameleon82 @Bouillie