Update pekko-stream, pekko-stream-testkit to 1.4.0 (#551)

Author: softwaremill-ci

build.sbt

@@ -9,7 +9,7 @@ val scala2 = List(scala2_12, scala2_13)
 val scala2alive = List(scala2_12, scala2_13)
 val scala3 = List("3.3.7")
 val akkaVersion = "2.6.20"
-val pekkoVersion = "1.3.0"
+val pekkoVersion = "1.4.0"
 val sttpModelVersion = "1.7.17"
 
 val scalaTestVersion = "3.2.19"