Make Envelope a reference type again (#6137)

* convert `Envelope` back into a reference type

* approved API changes

* changed to `readonly struct`

* fixed API approvals

src/core/Akka/Actor/Message.cs

@@ -12,7 +12,7 @@ namespace Akka.Actor
     /// <summary>
     /// Envelope class, represents a message and the sender of the message.
     /// </summary>
-    public struct Envelope
+    public readonly struct Envelope
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="Envelope"/> struct.
@@ -44,13 +44,13 @@ public Envelope(object message, IActorRef sender)
         /// Gets or sets the sender.
         /// </summary>
         /// <value>The sender.</value>
-        public IActorRef Sender { get; private set; }
+        public IActorRef Sender { get; }
 
         /// <summary>
         /// Gets or sets the message.
         /// </summary>
         /// <value>The message.</value>
-        public object Message { get; private set; }
+        public object Message { get; }
 
         /// <summary>
         /// Converts the <see cref="Envelope"/> to a string representation.