Modernize Smart Certificate KeyProvider

[anthonyadavisii]

• Replaced legacy certificate.PrivateKey usage with certificate.GetRSAPrivateKey() to align with current Windows cryptography APIs (CNG).
• Verified compatibility with x64 portable KeePass 2.60.
• Updated build configuration to target .NET Framework 4.8 and x64 architecture.
• Adjusted output path for direct deployment into KeePass’s Plugins folder.

Compatibility Notes

• New databases created with this plugin encrypt/decrypt successfully with pivkey smart cards. Additional hardware testing would be prudent.
• Legacy databases originally encrypted under CSP providers may not open on modern OS builds.

[anthonyadavisii]

I attempted fix in PR #15 in hope that it would resolve issues with legacy databases but it gave me the invalid provider error despite the very minor differences. This patch appears successful in encrypting / decrypting new databases but remains unable to decrypt legacy databases. It seems like the only alternative would be to decrypt on a Windows OS that supports the legacy CSPs but I have yet to attempt.