Update SDK to .NET 8 target net6.0 and net8.0, drop netcoreapp3.1, fix tests culture assert issues

[devlead]

Summary

Updates to target LTS .NET 8 and .NET 6, drops unsupported netcoreapp3.1, updates dependencies and transient dependencies needed for new target frameworks, and fixes so tests run on non-en-US operating systems.

Changes

• Add net6.0 and net8.0 moniker
• Update Microsoft.SourceLink.GitHub to 8.0.0
• Update SharpZipLib to 1.4.2 (security)
• Fix tests that can break on non en-US i.e. lunar indexes
• Update Microsoft.NET.Test.Sdk to ~~17.8.0~~ 17.9.0
• Update NUnit3TestAdapter to 4.5.0
• Update Microsoft.CodeAnalysis.CSharp.Scripting to 4.7.0
• Update Microsoft.CodeAnalysis.CSharp to 4.7.0
• Update System.Collections.Immutable to 7.0.0
• Address breaking change in code analysis
• Drop netcoreapp3.1, dropped by transient dependencies i.e. System.Reflection.MetaData, System.Text.Encoding, etc.
• Update Microsoft.Extensions.* to 6.0.x & 8.0.0
• Update Microsoft.AspNetCore.* to 6.0.25 & 8.0.0
• Update Microsoft.IO.RecyclableMemoryStream to 2.3.2
• Update Microsoft.Data.SqlClient to ~~5.1.2~~ 5.1.5 (security)
• Update NUnit to ~~3.14.0~~ 4.0.1
• Update Shouldly to 4.2.1