bump: kafka 1.21.0 (was 1.20.6) (#1840)

Author: scala-steward

build.sbt

@@ -27,7 +27,7 @@ val KafkaVersionForDocs = "39"
 // This should align with the ScalaTest version used in the Akka testkit
 // https://github.com/akka/akka/blob/main/project/Dependencies.scala#L44
 val scalatestVersion = "3.2.16"
-val testcontainersVersion = "1.20.6"
+val testcontainersVersion = "1.21.0"
 val slf4jVersion = "2.0.16"
 // this depends on Kafka, and should be upgraded to such latest version
 // that depends on the same Kafka version, as is defined above