Fix MNTR crashing because it is using PolyFill extension method (#6768)

* Revert "Make `HandleActorTaskSchedulerMessage` method `protected virtual` (#6763)"

This reverts commit 1345e58.

* Fix MNTR crashing because it is using PolyFill extension method

Author: Arkatufus

src/core/Akka.Remote.TestKit/CommandLine.cs

@@ -42,7 +42,7 @@ public static void Initialize(string[] args)
             var fixedArgs = new List<string>();
             for (var i = 1; i < args.Length - 1; ++i)
             {
-                if (args[i].Equals("-Dmultinode") && args[i + 1].StartsWith('.'))
+                if (args[i].Equals("-Dmultinode") && args[i + 1].StartsWith("."))
                 {
                     fixedArgs.Add(args[i] + args[i+1]);
                     ++i;

src/core/Akka/ActorState.cs

@@ -641,3 +641,4 @@ public Receive GetCurrentBehavior()
         }
     }
 }
+