Address deprecation warning when building vts under .NET v6

[scottprahl]

When building vts under .NET v6 I see

/Users/prahl/Documents/Code/git/vts/src/Vts/Range/RangeOfT.cs(132,50): warning CS0618: 
'Range<T>.AsEnumerable()' is obsolete: 'This method is deprecated. Use built-in IEnumerable implementation 
instead.' [/Users/prahl/Documents/Code/git/vts/src/Vts/Vts.csproj]

[hayakawa16]

This should be just a warning and not an error. When we remove a public class, we mark it as obsolete prior to removing it so that users have a chance to modify their code. Thanks for pointing that out so we can document our process.