- Add a dependency group for net8.0 to the nuspec

Some target frameworks declared in the dependencies group of the nuspec and the lib/ref folder do not have exact matches in the other location. Consult the list of actions below:

A stable release of a package should not have a prerelease dependency. Either modify the version spec of dependency "Microsoft.SemanticKernel.Plugins.Memory

Non-nullable property '_files' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.