fix timing regressions in Cluster.JoinAsync methods

We used the `cluster.seed-node-timeout` property incorrectly in #6033 - that is a "how long did it take from me to hear back from a seed" setting, but we're using it like a "how much time do I have to join the cluster?" setting.

Added a function designed to give us at least 20s of leeway.

src/core/Akka.Cluster/Cluster.cs

@@ -243,6 +243,16 @@ public void Join(Address address)
             ClusterCore.Tell(new ClusterUserAction.JoinTo(FillLocal(address)));
         }
 
+        /// <summary>
+        /// Want at least 10-20 seconds of leeway here.
+        /// </summary>
+        private TimeSpan ComputeJoinTimeLimit()
+        {
+            return TimeSpan.FromSeconds(Math.Max(
+                (Settings.RetryUnsuccessfulJoinAfter ?? TimeSpan.FromSeconds(10)).TotalSeconds,
+                TimeSpan.FromSeconds(20).TotalSeconds));
+        }
+
         /// <summary>
         /// Try to asynchronously join this cluster node specified by <paramref name="address"/>.
         /// A <see cref="Join"/> command is sent to the node to join. Returned task will be completed
@@ -273,7 +283,7 @@ public Task JoinAsync(Address address, CancellationToken token = default)
             var completion = new TaskCompletionSource<NotUsed>(TaskCreationOptions.RunContinuationsAsynchronously);
 
             var timeoutCts = CancellationTokenSource.CreateLinkedTokenSource(token);
-            timeoutCts.CancelAfter(Settings.SeedNodeTimeout);
+            timeoutCts.CancelAfter(ComputeJoinTimeLimit());
             timeoutCts.Token.Register(() =>
             {
                 timeoutCts.Dispose();
@@ -350,7 +360,7 @@ public Task JoinSeedNodesAsync(IEnumerable<Address> seedNodes, CancellationToken
             var nodes = seedNodes.ToList();
 
             var timeoutCts = CancellationTokenSource.CreateLinkedTokenSource(token);
-            timeoutCts.CancelAfter(Settings.SeedNodeTimeout);
+            timeoutCts.CancelAfter(ComputeJoinTimeLimit());
             timeoutCts.Token.Register(() =>
             {
                 timeoutCts.Dispose();