Update codeowners for System.Windows.* and System.Xaml.* to dotnet/wpf-developers (#4091)

Author: vatsan-madhavan

.github/CODEOWNERS

@@ -72,9 +72,9 @@
 /xml/System.Threading.Tasks**/** @tarekgh @stephentoub
 /xml/System.Transactions**/** @dasetser @HongGit
 /xml/System.Web**/** @scottaddie
-/xml/System.Windows**/** @vatsan-madhavan
+/xml/System.Windows**/** @dotnet/wpf-developers
 /xml/System.Windows.Forms**/** @RussKie @merriemcgaw
-/xml/System.Xaml**/** @vatsan-madhavan
+/xml/System.Xaml**/** @dotnet/wpf-developers
 /xml/System.Xml**/** @buyaa-n @krwq
 
 # System.Console