Use native AesGcm for .NET Framework from nuget

[scott-xu]

Based on dotnet/runtime#89718, we can use native AesGcm for .NET Framework from Microsoft.Bcl.Cryptography nuget package

[Rob-Hague]

LGTM once the package is stable

[Copilot]

Pull Request Overview

This PR enables the use of the native BCL AesGcm implementation for .NET Framework by adding the Microsoft.Bcl.Cryptography NuGet package. Previously, only .NET (Core) targets could use the native implementation, while .NET Framework and .NET Standard had to fall back to a BouncyCastle implementation.

Key changes:

• Changed conditional compilation from NET to !NETSTANDARD to include .NET Framework targets
• Added fallback handling for Mono environments that lack BCrypt.dll
• Added Microsoft.Bcl.Cryptography package reference for net462 target

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
AesGcmCipher.cs	Updated preprocessor directives and added Mono-specific exception handling for missing BCrypt.dll
AesGcmCipher.BclImpl.cs	Updated preprocessor directive to enable BCL implementation for .NET Framework
Renci.SshNet.csproj	Added package reference for Microsoft.Bcl.Cryptography for net462 target
Directory.Packages.props	Added version specification for Microsoft.Bcl.Cryptography package

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