document cluster bootstrap dependency on akka version

[alanngai]

the current documentation states that akka-management depends on akka 2.5 or later. However, when attempting to use akka management for cluster bootstrap (via async-dns resolver) with Lagom 1.4.5 (which depends on akka 2.5.12), we see a java.lang.NoClassDefFoundError: akka/io/dns/DnsProtocol$RequestType. Further research led to the conclusion that cluster bootstrap with async-dns resolver requires akka 2.5.14 or later. This should be made clear in the documentation.

[chbatey]

Thanks for reporting, want to raise a PR?

[TimMoore]

It's mentioned on Bootstrap recipes but should also be repeated on Kubernetes using DNS and the Discovery Method: DNS section, since not everyone will arrive at those pages via the recipes overview.

[raboof]

Also let's enforce it with https://github.com/akka/akka/blob/master/akka-actor/src/main/scala/akka/AkkaVersion.scala#L22 (though that will fail with a ClassNotFoundException before 2.5.9 ;) )