Add missing .ConfigureAwait(false) in HttpConnectionPool (#38610)

Author: stephentoub

src/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPool.cs

@@ -510,7 +510,7 @@ private ValueTask<HttpConnection> GetOrReserveHttp11ConnectionAsync(Cancellation
  }
 
  // If we reach this point, it means we need to fall back to a (new or existing) HTTP/1.1 connection.
- return await GetHttpConnectionAsync(request, cancellationToken);
+ return await GetHttpConnectionAsync(request, cancellationToken).ConfigureAwait(false);
  }
 
  public async Task<HttpResponseMessage> SendWithRetryAsync(HttpRequestMessage request, bool doRequestAuth, CancellationToken cancellationToken)