Removed usage of obsoleteted method in UserService.

src/Umbraco.Infrastructure/Services/Implement/UserService.cs

@@ -46,7 +46,7 @@ namespace Umbraco.Core.Services.Implement
         {
             using (var scope = ScopeProvider.CreateScope(autoComplete: true))
             {
-                return _userRepository.Exists(username);
+                return _userRepository.ExistsByUserName(username);
             }
         }