Add target for .NET 6/8 or .NET Standard 2.0/2.1

[DaveDezinski]

I'm using SaxonCS 12.4 which references ICU4N and ICU4N.Resources. The ICU4N.Resources NuGet package is targeting .NET Standard 1.6 which brings in a lot of out of date NuGet packages that contain security vulnerabilities and need updating. Any plans to update the NuGet package to add targets for .NET 6/8 or update the existing target to .NET Standard 2.0/2.1 so we can get rid of these old package references?