Added .NET 8 target

[multiarc]

1. Added .NET 8 Support

   • Aligned package versions with respective targets (.NET 6, netstandard2.0, .NET 8.0)
   • Removed .NET 7.0 target and outdated references
   • Removed direct targets on the library for .NET Framework and only left netstandard2.0 because of dependencies, netstandard2.0 reference required anyways and there is no point to split targets on the library itself.

2. Removed StyleCop.Analyzers in favor of Microsoft.CodeAnalysis.NetAnalyzers (debatable, but feels appropriate in this case); the reason being that StyleCop analyzers (even custom non-deprecated builds) are not always aware about newest C# features for .NET 8 target.

3. Fixed nullable references issues and other minor issues

   • Because struct is used as a base wrapper for everything it effectively made code incompatible with previous implementation when underlying value is any ValueType, because all input types became nullable and all output types became non-nullable. I'm not sure if making this breaking change is proper direction.

4. Moved packages for unit tests out of .props file

5. Upgraded xUnit

   • Fixed xUnit warnings

6. Upgraded the rest of packages in unit tests

7. Added packages packages.lock.json for repeatable restore