Use bounded shard rebalancing by default

[Aaronontheweb]

Summary

• set the default shard rebalance absolute limit to 20
• verify that the default configuration selects the bounded allocation strategy
• document the v1.6 behavior change and the explicit legacy compatibility setting

Validation

• dotnet test src/contrib/cluster/Akka.Cluster.Sharding.Tests/Akka.Cluster.Sharding.Tests.csproj -c Release --filter FullyQualifiedNameClusterShardingConfigSpec|FullyQualifiedNameLeastShardAllocationStrategySpec
• dotnet format src/contrib/cluster/Akka.Cluster.Sharding.Tests/Akka.Cluster.Sharding.Tests.csproj --verify-no-changes --no-restore (fails on existing unrelated formatting violations)

Refs #5488