Use System.Security.Cryptography in AesCipher (#1235)

* Use System.Security.Cryptography in AesCipher

* don't set the IV - it is unused by ECB

* Dispose ciphers in Session if applicable

---------

Co-authored-by: Wojciech Nagórski <wojtpl2@gmail.com>

Author: Rob-Hague

src/Renci.SshNet/PrivateKeyFile.cs

@@ -531,7 +531,14 @@ private static Key ParseOpenSshV1Key(byte[] keyFileData, string passPhrase)
                         throw new SshException("Cipher '" + cipherName + "' is not supported for an OpenSSH key.");
                 }
 
-                privateKeyBytes = cipher.Decrypt(privateKeyBytes);
+                try
+                {
+                    privateKeyBytes = cipher.Decrypt(privateKeyBytes);
+                }
+                finally
+                {
+                    cipher.Dispose();
+                }
             }
 
             // validate private key length