[csharp] fix ecdsa signing issue.

[Ghufz]

The current method ecdsa.SignHash() used for signing using ECDSA key, it returns the signature in ANS1 format and we have helper function ConvertToECDSAANS1Format() which convert ANS1 format to Der format.
We have observed that the function ConvertToECDSAANS1Format() returns invalid signature if we run it for more than 100 times.

To overcome this issue .NET5 has provided a method which can be used to generate signature for ecdsa key with DER format.

ecdsa.SignHash(dataToSign, DSASignatureFormat.Rfc3279DerSequence);

I have tested with new method it works as expected without any failure.

This method gives valid signature always and authentication happens without any fail.

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