!clt #17364 Make ShardRegion.props internal

* the extension api is enough
* simplify how roles are specified
* make proxy mode more explicit by seperate
  method `startProxy`

akka-cluster-sharding/src/multi-jvm/scala/akka/cluster/sharding/ClusterShardingLeavingSpec.scala

@@ -123,8 +123,8 @@ class ClusterShardingLeavingSpec extends MultiNodeSpec(ClusterShardingLeavingSpe
   def startSharding(): Unit = {
     ClusterSharding(system).start(
       typeName = "Entity",
-      entryProps = Some(Props[Entity]),
-      roleOverride = None,
+      entryProps = Props[Entity],
+      role = None,
       rememberEntries = false,
       idExtractor = idExtractor,
       shardResolver = shardResolver)