.NET 9 adds a conflicting version of `Index`

[atifaziz]

Index is landing in .NET 9. Unfortunately, it has an incompatible definition of the return value: Index in MoreLINQ returns a KeyValuePair<int, TSource> whereas Index in .NET returns (int Index, TSource Item). This means that the technique of merely hiding the extension method from a targeted build won't work for this.