Release notes in the Alpakka Kafka documentation

The Alpakka Kafka 2.1 series features

• Kafka 2.6.0 client
• Akka 2.6.x (dropped Akka 2.5)
• Scala 2.12 and 2.13 (dropped Scala 2.11)
• Testkit: use ScalaTest 3.1.x
• Testkit: default to Confluent Platform 6.0.0
• Testkit: no longer support Embedded Kafka (Kafka 2.6.0 can't be safely embedded in Scala applications)