Remove deprecated IWithBoundedStash

[ismaelhamed]

Supersedes #6511

Changes

• Removed deprecated IWithBoundedStashinterface.
• Refactored StashFactory and removed unnecessary AbstractStash implementations.
• Brought ActorWithStashSpec up-to-date with the JVM.

Checklist

• This change follows the Akka.NET API Compatibility Guidelines.
• I have reviewed my own pull request.
• Changes in public API reviewed, if any.

[ismaelhamed]

Creating an actor with bounded stash:

stash-capacity-mailbox {
  mailbox-type = "Akka.Dispatch.BoundedDequeBasedMailbox"
  stash-capacity = 50
}

...

public class StashCapacityActor : ReceiveActor, IWithStash
{
    public IStash Stash { get; set; }

    public static Props Props() => 
        Akka.Actor.Props.Create<StashCapacityActor>().WithMailbox("stash-capacity-mailbox");
}

or

public class StashCapacityActor : UntypedActorWithStash
{
    public static Props Props() => 
        Akka.Actor.Props.Create<StashCapacityActor>().WithMailbox("stash-capacity-mailbox");
}

[Aaronontheweb]

thanks @ismaelhamed - we'll take a look at this.

[Aaronontheweb]

I'm going to need to see if this breaks stashing support with Phobos or not - then I can review the rest of the PRs on its merits. I think it should be fine, but I need to make sure.

[Aaronontheweb]

Yeah this causes some compilation errors with Phobos - so I think what I'm going to do here is slate this for v1.6 (starting work on this in the fall.) I don't want to rock the boat with any additional breaking changes to v1.5 right now.