Remove warnings in akka-cluster (#26739)

Also undeprecate isTerminated but mark it internal

akka-cluster/src/multi-jvm/scala/akka/cluster/RestartNodeSpec.scala

@@ -25,6 +25,7 @@ import akka.testkit._
 import com.typesafe.config.ConfigFactory
 import akka.util.ccompat._
 
+@ccompatUsedUntil213
 object RestartNodeMultiJvmSpec extends MultiNodeConfig {
   val first = role("first")
   val second = role("second")
@@ -49,7 +50,7 @@ object RestartNodeMultiJvmSpec extends MultiNodeConfig {
       case ActorIdentity(None, Some(ref)) =>
         context.watch(ref)
         replyTo ! Done
-      case t: Terminated =>
+      case _: Terminated =>
     }
   }
 }