[HTTP] httpbin sometimes returns 503 (#128999)

Fix for httpbin returning 503, which is out of our control.
Based on Kusto, started 28.5., seen in
#128940

Author: ManickaP

src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.RemoteServer.cs

@@ -1290,7 +1290,7 @@ public async Task GetAsync_SetAutomaticDecompression_ContentDecompressed_Deflate
                 try
                 {
                     using HttpResponseMessage response = await client.GetAsync(uri);
-                    if (response.StatusCode is HttpStatusCode.GatewayTimeout or HttpStatusCode.BadGateway)
+                    if (response.StatusCode is HttpStatusCode.GatewayTimeout or HttpStatusCode.BadGateway or HttpStatusCode.ServiceUnavailable)
                     {
                         // Ignore the erroneous status code, the test depends on an external server that is out of our control.
                         _output.WriteLine(response.ToString());