Native: mark NamedCurve enum as deprecated

It will be replaced by NamedCurveHash in the next commit.

A part of neo-project/neo#3209.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

src/Neo.SmartContract.Framework/Native/NamedCurve.cs

@@ -8,17 +8,24 @@
 // Redistribution and use in source and binary forms with or without
 // modifications are permitted.
 
+using System;
+
 namespace Neo.SmartContract.Framework.Native
 {
     /// <summary>
-    /// RFC 4492
+    /// Represents the named curve (RFC 4492) used in ECDSA. This enum is obsolete
+    /// and will be removed in future versions. Please, use an extended compatible
+    /// <see cref="NamedCurveHash"/> instead.
     /// </summary>
     /// <remarks>
     /// https://tools.ietf.org/html/rfc4492#section-5.1.1
     /// </remarks>
+    [Obsolete("NamedCurve enum is obsolete and will be removed in future versions. Please, use an extended compatible NamedCurveHash instead.")]
     public enum NamedCurve : byte
     {
+        [Obsolete("secp256k1 value is obsolete and will be removed in future versions. Please, use NamedCurveHash.secp256k1SHA256 for compatible behaviour.")]
         secp256k1 = 22,
+        [Obsolete("secp256r1 value is obsolete and will be removed in future versions. Please, use NamedCurveHash.secp256r1SHA256 for compatible behaviour.")]
         secp256r1 = 23
     }
 }