Fixed: Akka.Cluster.Singleton no longer moves when node with matching role and higher `AppVersion` joins cluster

[Aaronontheweb]

Changes

Fixes #7196 - not done yet.

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• [x] This change follows the Akka.NET API Compatibility Guidelines.
• [x] This change follows the Akka.NET Wire Compatibility Guidelines.
• [x] I have reviewed my own pull request.
• [x] Design discussion issue #7196
• [x] Changes in public API reviewed, if any.

[Aaronontheweb]

I have a racy failure / instability of some kind in my test. Working on ruling that out before I get anyone else to look at this PR. I think it might have been related to the auto-downing setting I copied and pasted from the Restart2Spec.

[Aaronontheweb]

So this "fix" will actually create additional problems - I'm thinking that the right thing to do here is to remove the "consider app version" support from Akka.Cluster.Singleton altogether. That problem doesn't seem workable without adding another communication layer around singletons, which I am not inclined to do. Better that we have a single heuristic for deciding where singletons live so we can keep the design of the infrastructure as simple as possible and eliminate any possibility of split brains.

[Aaronontheweb]

Superseded by https://github.com/akkadotnet/akka.net/pull/7297