Readd netstandard 2.0

[adamhathcock]

addresses later comments on #1048

This pull request expands support for the SharpCompress library by adding the netstandard2.0 target framework. The changes ensure compatibility with more .NET platforms, update dependency management, and adjust conditional logic to include the new framework where appropriate.

Framework support and compatibility:

• Added netstandard2.0 to the TargetFrameworks property in SharpCompress.csproj, enabling builds for .NET Standard 2.0.
• Updated conditional logic in WinzipAesEncryptionData.cs to include NETSTANDARD2_0 alongside NETFRAMEWORK for password-based key derivation.

Dependency management:

• Modified package references in SharpCompress.csproj so that Microsoft.Bcl.AsyncInterfaces, System.Text.Encoding.CodePages, and System.Buffers are included for both net48 and netstandard20 targets.
• Added a new section for .NETStandard,Version=v2.0 in packages.lock.json, specifying direct and transitive dependencies required for this framework.

[Copilot]

Pull request overview

This pull request adds .NET Standard 2.0 target framework support to the SharpCompress library, expanding platform compatibility. The changes update the project configuration, adjust conditional compilation directives for cryptographic operations, and establish the necessary dependency declarations.

Key Changes:

• Added netstandard20 to the list of target frameworks alongside net48, net8.0, and net10.0
• Updated conditional compilation in WinzipAesEncryptionData.cs to use the appropriate Rfc2898DeriveBytes API for .NET Standard 2.0
• Extended package dependencies to include netstandard2.0 alongside net48 for compatibility packages

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/SharpCompress/SharpCompress.csproj	Added netstandard20 to TargetFrameworks; updated conditional package references to include netstandard20; removed VersionPrefix, AssemblyVersion, and FileVersion properties
src/SharpCompress/Common/Zip/WinzipAesEncryptionData.cs	Updated conditional compilation directive to include NETSTANDARD2_0 for password-based key derivation using legacy Rfc2898DeriveBytes constructor
src/SharpCompress/packages.lock.json	Added complete dependency tree for .NETStandard,Version=v2.0 framework including required compatibility packages

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