Update README: add badges for .NET Standard 2.1, .NET 6, and .NET 7

Author: 0x25CBFC4F

README.md

@@ -5,6 +5,9 @@ A modern, cross-platform .NET library providing managed bindings for libssh2, en
 [![NuGet](https://img.shields.io/nuget/v/NullOpsDevs.LibSsh)](https://www.nuget.org/packages/NullOpsDevs.LibSsh/)
 [![NuGet](https://img.shields.io/nuget/vpre/NullOpsDevs.LibSsh)](https://www.nuget.org/packages/NullOpsDevs.LibSsh/)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](LICENSE)
+[![.NET](https://img.shields.io/badge/.NET-Standard_2.1-purple.svg)](https://dotnet.microsoft.com/)
+[![.NET](https://img.shields.io/badge/.NET-6.0-purple.svg)](https://dotnet.microsoft.com/)
+[![.NET](https://img.shields.io/badge/.NET-7.0-purple.svg)](https://dotnet.microsoft.com/)
 [![.NET](https://img.shields.io/badge/.NET-8.0-purple.svg)](https://dotnet.microsoft.com/)
 [![.NET](https://img.shields.io/badge/.NET-9.0-purple.svg)](https://dotnet.microsoft.com/)
 [![Documentation](https://img.shields.io/badge/Documentation-blue)](https://libsshnet.nullops.systems/quickstart)