Deprecate EnumFlagsAttribute

Author: keveleigh

org.mixedrealitytoolkit.core/Utilities/Attributes/EnumFlagsAttribute.cs

@@ -14,11 +14,12 @@ namespace MixedReality.Toolkit
     /// From https://answers.unity.com/questions/486694/default-editor-enum-as-flags-.html
     /// </remarks>
     [AttributeUsage(AttributeTargets.Field)]
+    [Obsolete("Use Unity.XR.CoreUtils.GUI.FlagsPropertyAttribute instead.")]
     public sealed class EnumFlagsAttribute : PropertyAttribute
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="EnumFlagsAttribute"/> class.
         /// </summary>
         public EnumFlagsAttribute() { }
     }
-}
+}