Split lock to prevent deadlock between OnWindowsUpdate and Complete.

Http2Connection.ChangeInitialWindowSize locks connection's SyncObject and calls Http2Stream.OnWindowUpdate which locks stream's SyncObject.
Http2Stream.Complete is called only while stream's SyncObject lock is take and then it calls Http2Connection.RemoveStream that locks connection SyncObject.

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http2Stream.cs

@@ -39,6 +39,7 @@ private sealed class Http2Stream : IValueTaskSource, IHttpHeadersHandler, IHttpT
             private int _pendingWindowUpdate;
             private CreditWaiter? _creditWaiter;
             private int _availableCredit;
+            private readonly object _creditSyncObject = new object(); // split from SyncObject to mitigate deadlock in OnWindowUpdate.
 
             private StreamCompletionState _requestCompletionState;
             private StreamCompletionState _responseCompletionState;
@@ -349,11 +350,14 @@ private void Complete()
 
                 _connection.RemoveStream(this);
 
-                CreditWaiter? w = _creditWaiter;
-                if (w != null)
+                lock (_creditSyncObject)
                 {
-                    w.Dispose();
-                    _creditWaiter = null;
+                    CreditWaiter? waiter = _creditWaiter;
+                    if (waiter != null)
+                    {
+                        waiter.Dispose();
+                        _creditWaiter = null;
+                    }
                 }
             }
 
@@ -421,7 +425,7 @@ private void Cancel()
 
             public void OnWindowUpdate(int amount)
             {
-                lock (SyncObject)
+                lock (_creditSyncObject)
                 {
                     _availableCredit = checked(_availableCredit + amount);
                     if (_availableCredit > 0 && _creditWaiter != null)
@@ -1219,7 +1223,7 @@ private async ValueTask SendDataAsync(ReadOnlyMemory<byte> buffer, CancellationT
                     while (buffer.Length > 0)
                     {
                         int sendSize = -1;
-                        lock (SyncObject)
+                        lock (_creditSyncObject)
                         {
                             if (_availableCredit > 0)
                             {