Enable netstandard2.1 build

FEATURE_ECDSA catches netstandard1.3, which doesnt have ECDSA Support.
So enable it for all other netstandard versions.

Author: darinkes

src/Renci.SshNet/Security/Cryptography/EcdsaDigitalSignature.cs

@@ -39,7 +39,7 @@ public override bool Verify(byte[] input, byte[] signature)
             // for 521 sig_size is 132
             var sig_size = _key.KeyLength == 521 ? 132 : _key.KeyLength / 4;
             var ssh_data = new SshDataSignature(signature, sig_size);
-#if NETSTANDARD2_0
+#if NETSTANDARD
             return _key.Ecdsa.VerifyData(input, ssh_data.Signature, _key.HashAlgorithm);
 #else
             var ecdsa = (ECDsaCng)_key.Ecdsa;
@@ -57,7 +57,7 @@ public override bool Verify(byte[] input, byte[] signature)
         /// </returns>
         public override byte[] Sign(byte[] input)
         {
-#if NETSTANDARD2_0
+#if NETSTANDARD
             var signed = _key.Ecdsa.SignData(input, _key.HashAlgorithm);
 #else
             var ecdsa = (ECDsaCng)_key.Ecdsa;

src/Renci.SshNet/Security/Cryptography/EcdsaKey.cs

@@ -18,7 +18,7 @@ public class EcdsaKey : Key, IDisposable
         internal const string ECDSA_P384_OID_VALUE = "1.3.132.0.34"; // Also called nistP384 or secP384r1
         internal const string ECDSA_P521_OID_VALUE = "1.3.132.0.35"; // Also called nistP521or secP521r1
 
-#if !NETSTANDARD2_0
+#if !NETSTANDARD
         internal enum KeyBlobMagicNumber : int
         {
             BCRYPT_ECDSA_PUBLIC_P256_MAGIC = 0x31534345,
@@ -57,7 +57,7 @@ public override string ToString()
             return string.Format("ecdsa-sha2-nistp{0}", KeyLength);
         }
 
-#if NETSTANDARD2_0
+#if NETSTANDARD
         /// <summary>
         /// Gets the HashAlgorithm to use
         /// </summary>
@@ -144,7 +144,7 @@ public override BigInteger[] Public
                 byte[] curve;
                 byte[] qx;
                 byte[] qy;
-#if NETSTANDARD2_0
+#if NETSTANDARD
                 var parameter = Ecdsa.ExportParameters(false);
                 qx = parameter.Q.X;
                 qy = parameter.Q.Y;
@@ -283,7 +283,7 @@ public EcdsaKey(byte[] data)
 
         private void Import(string curve_oid, byte[] publickey, byte[] privatekey)
         {
-#if NETSTANDARD2_0
+#if NETSTANDARD
             var curve = ECCurve.CreateFromValue(curve_oid);
             var parameter = new ECParameters
             {