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 17.11.1
• Update NUnit3TestAdapter to 4.5.0 4.6.0
• 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 6.0.33 & 8.0.x
• 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
• Update SixLabors.ImageSharp to 2.1.9 (security)

[daveaglick]

This is great, thanks! Retargeting was kind of on my "last thing to do before releasing 1.0" mind, so having the groundwork done hopefully helps (finally!) get 1.0 out the door. Will look at merging this ASAP(ish).

[devlead]

Excellent 👍