Set SocketsHttpHandler's default connect timeout to 15s (dotnet#66607)

* Set default connect timeout to 15s

* Fix ConnectTimeout_Default test

Author: CarnaViire

src/libraries/Common/src/System/Net/Http/HttpHandlerDefaults.cs

@@ -27,6 +27,6 @@ internal static partial class HttpHandlerDefaults
         public static readonly TimeSpan DefaultPooledConnectionLifetime = Timeout.InfiniteTimeSpan;
         public static readonly TimeSpan DefaultPooledConnectionIdleTimeout = TimeSpan.FromMinutes(1);
         public static readonly TimeSpan DefaultExpect100ContinueTimeout = TimeSpan.FromSeconds(1);
-        public static readonly TimeSpan DefaultConnectTimeout = Timeout.InfiniteTimeSpan;
+        public static readonly TimeSpan DefaultConnectTimeout = TimeSpan.FromSeconds(15);
     }
 }

src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.cs

@@ -1054,7 +1054,7 @@ public void ConnectTimeout_Default()
         {
             using (var handler = new SocketsHttpHandler())
             {
-                Assert.Equal(Timeout.InfiniteTimeSpan, handler.ConnectTimeout);
+                Assert.Equal(TimeSpan.FromSeconds(15), handler.ConnectTimeout);
             }
         }