Introduce warning silencer plugin (#26588)

So now we can compile akka-distributed-data with
-Xfatal-warnings - though I'm not yet sure about
enabling the (other) undisciplineScalacOptions

* Fix multi-node silencing
* Fix scaladoc warnings
* Introduce annotation to declare ccompat use
* Add explicit toString
* Fix deprecation on 2.13
* Move 'immutable' ccompat helpers to shared ccompat package
* Add MiMa for internal scala 2.13 compatibility class
* Internal API markers
* Fix scaladoc generation
Got bitten by https://github.com/scala/bug/issues/11021

akka-distributed-data/src/test/scala/akka/cluster/ddata/LotsOfDataBot.scala

@@ -24,7 +24,7 @@ object LotsOfDataBot {
     if (args.isEmpty)
       startup(Seq("2551", "2552", "0"))
     else
-      startup(args)
+      startup(args.toIndexedSeq)
   }
 
   def startup(ports: Seq[String]): Unit = {