Need to make AkkaManagementSetup and ClusterBootstrapSetup immutable

[Arkatufus]

These Setup classes introduced in #730 and #731 are mutable right now, will need to make them immutable in the future, most probably by converting them to record when we switch to .NET 6 fully.

[Aaronontheweb]

Going to assume this is something that'll happen around Akka.NET 2.0