Disable formatter test on Mono: COM support is disabled on Mono runti…

…me. (#56806)

This particular test is using a COM object, but COM support is
disabled on Mono runtime.

src/libraries/System.Runtime.Serialization.Formatters/tests/FormatterServicesTests.Windows.cs

@@ -11,7 +11,7 @@ namespace System.Runtime.Serialization.Formatters.Tests
     public partial class FormatterServicesTests
     {
         [Fact]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/39704", TestRuntimes.Mono)]
+        [SkipOnMono("COM support is disabled on Mono runtime. Refer to issue #39704.")]
         public void GetUninitializedObject_COMObject_ThrowsNotSupportedException()
         {
             Type comObjectType = typeof(COMObject);